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42 results on '"correlated variability"'

1. Numerical characterization of support recovery in sparse regression with correlated design

Author

Kumar, Ankit, Bhattacharyya, Sharmodeep, and Bouchard, Kristofer

Subjects
Information and Computing Sciences, Computer Vision and Multimedia Computation, Correlated variability, Model selection, Sparse regression, Information criteria, Compressed sensing, Mathematical Sciences, Statistics & Probability, Information and computing sciences, Mathematical sciences
Published
2024

2. The geometry of correlated variability leads to highly suboptimal discriminative sensory coding.

Author

Livezey, Jesse A., Sachdeva, Pratik S., Dougherty, Maximilian E., Summers, Mathew T., and Bouchard, Kristofer E.

Subjects
*NEUROPHYSIOLOGY, *GEOMETRY, *NOISE, *STIMULUS & response (Psychology)
Abstract

The brain represents the world through the activity of neural populations; however, whether the computational goal of sensory coding is to support discrimination of sensory stimuli or to generate an internal model of the sensory world is unclear. Correlated variability across a neural population (noise correlations) is commonly observed experimentally, and many studies demonstrate that correlated variability improves discriminative sensory coding compared to a null model with no correlations. However, such results do not address whether correlated variability is optimal for discriminative sensory coding. If the computational goal of sensory coding is discriminative, than correlated variability should be optimized to support that goal. We assessed optimality of noise correlations for discriminative sensory coding in diverse datasets by developing two novel null models, each with a biological interpretation. Across datasets, we found that correlated variability in neural populations leads to highly suboptimal discriminative sensory coding according to both null models. Furthermore, biological constraints prevent many subsets of the neural populations from achieving optimality, and subselecting based on biological criteria leaves red discriminative coding performance suboptimal. Finally, we show that optimal subpopulations are exponentially small as the population size grows. Together, these results demonstrate that the geometry of correlated variability leads to highly suboptimal discriminative sensory coding. NEW & NOTEWORTHY: The brain represents the world through the activity of neural populations that exhibit correlated variability. We assessed optimality of correlated variability for discriminative sensory coding in diverse datasets by developing two novel null models. Across datasets, correlated variability in neural populations leads to highly suboptimal discriminative sensory coding according to both null models. Biological constraints prevent the neural populations from achieving optimality. Together, these results demonstrate that the geometry of correlated variability leads to highly suboptimal discriminative sensory coding. [ABSTRACT FROM AUTHOR]

Published
2025
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3. Recent Visual Experience Reshapes V4 Neuronal Activity and Improves Perceptual Performance.

Author

Stan, Patricia L. and Smith, Matthew A.

Subjects
*DIFFERENTIATION (Cognition), *VISUAL perception, *FORM perception, *SIGNAL separation, *RHESUS monkeys
Abstract

Recent visual experience heavily influences our visual perception, but how neuronal activity is reshaped to alter and improve perceptual discrimination remains unknown. We recorded from populations of neurons in visual cortical area V4 while two male rhesus macaque monkeys performed a natural image change detection task under different experience conditions. We found that maximizing the recent experience with a particular image led to an improvement in the ability to detect a change in that image. This improvement was associated with decreased neural responses to the image, consistent with neuronal changes previously seen in studies of adaptation and expectation. We found that the magnitude of behavioral improvement was correlated with the magnitude of response suppression. Furthermore, this suppression of activity led to an increase in signal separation, providing evidence that a reduction in activity can improve stimulus encoding. Within populations of neurons, greater recent experience was associated with decreased trial-to-trial shared variability, indicating that a reduction in variability is a key means by which experience influences perception. Taken together, the results of our study contribute to an understanding of how recent visual experience can shape our perception and behavior through modulating activity patterns in the mid-level visual cortex. [ABSTRACT FROM AUTHOR]

Published
2024
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5. Insights into decision making using choice probability

Author

Crapse, Trinity B and Basso, Michele A

Subjects
Biomedical and Clinical Sciences, Neurosciences, Brain Disorders, Basic Behavioral and Social Science, Behavioral and Social Science, Neurological, Animals, Brain, Choice Behavior, Feedback, Physiological, Humans, Models, Neurological, Probability, signal detection theory, sensation, perception, neurophysiology, vision, eye movements, correlated variability, multiple neuron recording, Medical and Health Sciences, Psychology and Cognitive Sciences, Neurology & Neurosurgery, Biomedical and clinical sciences, Health sciences, Psychology
Abstract

A long-standing question in systems neuroscience is how the activity of single neurons gives rise to our perceptions and actions. Critical insights into this question occurred in the last part of the 20th century when scientists began linking modulations of neuronal activity directly to perceptual behavior. A significant conceptual advance was the application of signal detection theory to both neuronal activity and behavior, providing a quantitative assessment of the relationship between brain and behavior. One metric that emerged from these efforts was choice probability (CP), which provides information about how well an ideal observer can predict the choice an animal makes from a neuron's discharge rate distribution. In this review, we describe where CP has been studied, locational trends in the values found, and why CP values are typically so low. We discuss its dependence on correlated activity among neurons of a population, assess whether it arises from feedforward or feedback mechanisms, and investigate what CP tells us about how many neurons are required for a decision and how they are pooled to do so.

Published
2015

6. Population Coding of Natural Electrosensory Stimuli by Midbrain Neurons.

Author

Metzen, Michael G. and Chacron, Maurice J.

Subjects
*MESENCEPHALON, *STIMULUS & response (Psychology), *NEURONS, *SENSORY neurons, *INFERIOR colliculus
Abstract

Natural stimuli display spatiotemporal characteristics that typically vary over orders of magnitude, and their encoding by sensory neurons remains poorly understood. We investigated population coding of highly heterogeneous natural electrocommunication stimuli in Apteronotus leptorhynchus of either sex. Neuronal activities were positively correlated with one another in the absence of stimulation, and correlation magnitude decayed with increasing distance between recording sites. Under stimulation, we found that correlations between trial-averaged neuronal responses (i.e., signal correlations) were positive and higher in magnitude for neurons located close to another, but that correlations between the trial-to-trial variability (i.e., noise correlations) were independent of physical distance. Overall, signal and noise correlations were independent of stimulus waveform as well as of one another. To investigate how neuronal populations encoded natural electrocommunication stimuli, we considered a nonlinear decoder for which the activities were combined. Decoding performance was best for a timescale of 6 ms, indicating that midbrain neurons transmit information via precise spike timing. A simple summation of neuronal activities (equally weighted sum) revealed that noise correlations limited decoding performance by introducing redundancy. Using an evolution algorithm to optimize performance when considering instead unequally weighted sums of neuronal activities revealed much greater performance values, indicating that midbrain neuron populations transmit information that reliably enable discrimination between different stimulus waveforms. Interestingly, we found that different weight combinations gave rise to similar discriminability, suggesting robustness. Our results have important implications for understanding how natural stimuli are integrated by downstream brain areas to give rise to behavioral responses. [ABSTRACT FROM AUTHOR]

Published
2021
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7. The impact of correlated variability on models of neural coding

Author

Sachdeva, Pratik Singh

Subjects
Neurosciences, computational neuroscience, correlated variability, machine learning, neural coding, parametric modeling, systems neuroscience
Abstract

Variability is a prominent feature of neural systems: neural responses to repeated presentations of the same external stimulus will typically vary from trial to trial. Furthermore, neural variability exhibits pairwise correlations, commonly referred to as correlated variability. Correlated variability is a pervasive neural phenomenon that arises due to a variety of sources including shared input, biological noise, global fluctuations, and neural activity unobserved by experimental apparatuses. It is of theoretical interest because of its importance for models of neural coding: the existence of correlated variability can improve or harm neural coding depending on its structure. In this work, we examine how correlated variability impacts neural coding for both analyses on decoding efficacy and parametric models of neural activity. First, we demonstrate that correlated variability induced by noise sources common to a neural population can be manipulated by heterogeneous synaptic weighting to improve neural coding, even at the cost of amplifying the noise. Second, we demonstrate that correlated variability in neural data exhibits worse than chance decoding fidelity, and identify biological constraints in achieving optimal neural representations. Third, we examine how an improved inference algorithm for common parametric models can shape the scientific interpretation of common systems neuroscience models, despite the presence of correlated variability in the data. Lastly, we identify how omitting correlated variability arising from unobserved activity in parametric models of tuning and functional coupling can bias parametric estimates, and propose a new model and inference procedure to mitigate these biases. Together, our results highlight the importance of correlated variability on a wide range neural coding models.

Published
2021

8. Mrk 421 active state in 2008: the MAGIC view, simultaneous multi-wavelength observations and SSC model constrained

Author

Antoranz Canales, Pedro, Barrio Uña, Juan Abel, Contreras González, José Luis, Fonseca González, Mª Victoria, López Moya, Marcos, Miranda Pantoja, José Miguel, Nieto, Daniel, Scapin, Valeria, Antoranz Canales, Pedro, Barrio Uña, Juan Abel, Contreras González, José Luis, Fonseca González, Mª Victoria, López Moya, Marcos, Miranda Pantoja, José Miguel, Nieto, Daniel, and Scapin, Valeria

Abstract

© ESO. We would like to thank the Instituto de Astrofisica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN, the Swiss National Fund SNF, and the Spanish MICINN is gratefully acknowledged. This work was also supported by the Marie Curie program, by the CPAN CSD2007-00042 and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 programme, by grant DO02-353 of the Bulgarian NSF, by grant 127740 of the Academy of Finland, by the YIP of the Helmholtz Gemeinschaft, by the DFG Cluster of Excellence "Origin and Structure of the Universe", and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0. We acknowledge insightful and constructive criticism from the anonymous referee., The blazar Markarian 421 is one of the brightest TeV gamma-ray sources of the northern sky. From December 2007 until June 2008 it was intensively observed in the very high energy (VHE, E > 100 GeV) band by the single-dish Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC-I). Aims. We aimed to measure the physical parameters of the emitting region of the blazar jet during active states. Methods. We performed a dense monitoring of the source in VHE with MAGIC-I, and also collected complementary data in soft X-rays and optical-UV bands; then, we modeled the spectral energy distributions (SED) derived from simultaneous multi-wavelength data within the synchrotron self-Compton (SSC) framework. Results. The source showed intense and prolonged.-ray activity during the whole period, with integral fluxes (E > 200 GeV) seldom below the level of the Crab Nebula, and up to 3.6 times this value. Eight datasets of simultaneous optical-UV (KVA, Swift/UVOT), soft X-ray (Swift/XRT) and MAGIC-I VHE data were obtained during different outburst phases. The data constrain the physical parameters of the jet, once the spectral energy distributions obtained are interpreted within the framework of a single-zone SSC leptonic model. Conclusions. The main outcome of the study is that within the homogeneous model high Doppler factors (40 <= delta <= 80) are needed to reproduce the observed SED; but this model cannot explain the observed short time-scale variability, while it can be argued that inhomogeneous models could allow for less extreme Doppler factors, more intense magnetic fields and shorter electron cooling times compatible with hour or sub-hour scale variability., German BMBF, MPG, Italian INFN, Swiss National Fund SNF, Spanish MICINN, Marie Curie program, Spanish Consolider-Ingenio, Bulgarian NSF, Academy of Finland, YIP of the Helmholtz Gemeins, Polish MNiSzW, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

Published
2023

9. MAGIC TeV gamma-ray observations of Markarian 421 during multiwavelength campaigns in 2006

Author

Antoranz Canales, Pedro, Barrio Uña, Juan Abel, Contreras González, José Luis, Fonseca González, Mª Victoria, Miranda Pantoja, José Miguel, Nieto, Daniel, Antoranz Canales, Pedro, Barrio Uña, Juan Abel, Contreras González, José Luis, Fonseca González, Mª Victoria, Miranda Pantoja, José Miguel, and Nieto, Daniel

Abstract

© ESO 2010. We thank the Instituto de Astrofisica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN and Spanish MICINN is gratefully acknowledged. This work was also supported by ETH Research Grant TH 34/043, by the Polish MNiSzW Grant N N203 390834, and by the YIP of the Helmholtz Gemeinschaft., Context. Wide-range spectral coverage of blazar-type active galactic nuclei is of paramount importance for understanding the particle acceleration mechanisms assumed to take place in their jets. The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn) 421 during the nights of April 28 and 29, 2006, and June 14, 2006. Aims. We analyzed the corresponding MAGIC very-high energy observations during 9 nights from April 22 to 30, 2006 and on June 14, 2006. We inferred light curves with sub-day resolution and night-by-night energy spectra. Methods. MAGIC detects gamma-rays by observing extended air showers in the atmosphere. The obtained air-shower images were analyzed using the standard MAGIC analysis chain. Results. A strong gamma-ray signal was detected from Mkn 421 on all observation nights. The flux (E > 250 GeV) varied on night-by-night basis between (0.92 +/- 0.11) x 10(-10) cm(-2) s(-1) (0.57 Crab units) and (3.21 +/- 0.15) x 10(-10) cm(-2) s(-1) (2.0 Crab units) in April 2006. There is a clear indication for intra-night variability with a doubling time of 36 +/- 10(stat) min on the night of April 29, 2006, establishing once more rapid flux variability for this object. For all individual nights gamma-ray spectra could be inferred, with power-law indices ranging from 1.66 to 2.47. We did not find statistically significant correlations between the spectral index and the flux state for individual nights. During the June 2006 campaign, a flux substantially lower than the one measured by the Whipple 10-m telescope four days later was found. Using a log-parabolic power law fit we deduced for some data sets the location of the spectral peak in the very-high energy regime. Our results confirm the indications of rising peak energy with increasing flux, as expected in leptonic acceleration models., German BMBF, German MPG, Italian INFN, Spanish MICINN, ETH, Polish MNiSzW, YIP of the Helmholtz Gemeinschaft, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

Published
2023

10. Simultaneous multiwavelength observations of Markarian 421 during outburst

Author

Antoranz Canales, Pedro, Barrio Uña, Juan Abel, Contreras González, José Luis, Fonseca González, Mª Victoria, Miranda Pantoja, José Miguel, Nieto, Daniel, Antoranz Canales, Pedro, Barrio Uña, Juan Abel, Contreras González, José Luis, Fonseca González, Mª Victoria, Miranda Pantoja, José Miguel, and Nieto, Daniel

Abstract

© The American Astronomical Society. The VERITAS research was supported by grants from the U.S. Department of Energy, the U.S. National Science Foundation and the Smithsonian Institution, by NSERC in Canada, by Science Foundation Ireland, and by STFC in the U.K. We acknowledge the excellent work of the technical support staff at the FLWO and the collaborating institutions in the construction and operation of the instrument. D. G. and W. C. wish to acknowledge support by NASA through grants NNX06AB96G, NNX08AD76G, and NNX08AX53G. The MAGIC Collaboration thanks the Instituto de Astrofisica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The support of the German BMBF and MPG, the Italian INFN, and Spanish MCINN is gratefully acknowledged. This work was also supported by ETH Research Grant TH 34/043, by the Polish MNiSzW Grant N N203 390834, and by the YIP of the Helmholtz Gemeinschaft. Facilities: MAGIC, VERITAS, FLWO:10m, XMM, We report on the results of two coordinated multiwavelength campaigns that focused on the blazar Markarian 421 during its 2006 and 2008 outbursts. These campaigns obtained UV and X-ray data using the XMM-Newton satellite, while the gamma-ray data were obtained utilizing three imaging atmospheric Cerenkov telescopes, the Whipple 10 m telescope and VERITAS, both based in Arizona, as well as the MAGIC telescope, based on La Palma in the Canary Islands. The coordinated effort between the gamma-ray groups allowed for truly simultaneous data in UV/X-ray/gamma-ray wavelengths during a significant portion of the XMM-Newton observations. This simultaneous coverage allowed for a reliable search for correlations between UV, X-ray, and gamma-ray variability over the course of the observations. Investigations of spectral hysteresis and modeling of the spectral energy distributions are also presented., U.S. Department of Energy, U.S. National Science Foundation, Smithsonian Institution, NSERC in Canada, Science Foundation Ireland, STFC, U.K, NASA, ETH, Polish MNiSzW, Helmholtz Gemeinschaft, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

Published
2023

11. Population Coding and Correlated Variability in Electrosensory Pathways

Author

Volker Hofmann and Maurice J. Chacron

Subjects
population coding, correlated variability, noise correlations, stimulus encoding, feedback, electric fish, Neurosciences. Biological psychiatry. Neuropsychiatry, RC321-571, Neurology. Diseases of the nervous system, RC346-429
Abstract

The fact that perception and behavior depend on the simultaneous and coordinated activity of neural populations is well established. Understanding encoding through neuronal population activity is however complicated by the statistical dependencies between the activities of neurons, which can be present in terms of both their mean (signal correlations) and their response variability (noise correlations). Here, we review the state of knowledge regarding population coding and the influence of correlated variability in the electrosensory pathways of the weakly electric fish Apteronotus leptorhynchus. We summarize known population coding strategies at the peripheral level, which are largely unaffected by noise correlations. We then move on to the hindbrain, where existing data from the electrosensory lateral line lobe (ELL) shows the presence of noise correlations. We summarize the current knowledge regarding the mechanistic origins of noise correlations and known mechanisms of stimulus dependent correlation shaping in ELL. We finish by considering future directions for understanding population coding in the electrosensory pathways of weakly electric fish, highlighting the benefits of this model system for understanding the origins and impact of noise correlations on population coding.

Published
2018
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12. Population Coding and Correlated Variability in Electrosensory Pathways.

Author

Hofmann, Volker and Chacron, Maurice J.

Abstract

The fact that perception and behavior depend on the simultaneous and coordinated activity of neural populations is well established. Understanding encoding through neuronal population activity is however complicated by the statistical dependencies between the activities of neurons, which can be present in terms of both their mean (signal correlations) and their response variability (noise correlations). Here, we review the state of knowledge regarding population coding and the influence of correlated variability in the electrosensory pathways of the weakly electric fish Apteronotus leptorhynchus. We summarize known population coding strategies at the peripheral level, which are largely unaffected by noise correlations. We then move on to the hindbrain, where existing data from the electrosensory lateral line lobe (ELL) shows the presence of noise correlations. We summarize the current knowledge regarding the mechanistic origins of noise correlations and known mechanisms of stimulus dependent correlation shaping in ELL. We finish by considering future directions for understanding population coding in the electrosensory pathways of weakly electric fish, highlighting the benefits of this model system for understanding the origins and impact of noise correlations on population coding. [ABSTRACT FROM AUTHOR]

Published
2018
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14. Brain as a Complex System, harnessing systems neuroscience tools & notions for an empirical approach

Author

Safavi, Shervin and Logothetis, Nikos K. (Prof. Dr.)

Subjects
prefrontal cortex, neural network modeling, efficient coding, neural events, visual perception, binocular rivalry, spike-LFP coupling, multi-scale analysis, statistical analysis, correlated variability, criticality, noise correlation, complex systems, complex adaptive systems
Abstract

The brain can be conceived as a complex system, as it is made up of nested networks of interactions and moreover, demonstrates emergent-like behaviors such as oscillations. Based on this conceptualization, various tools and frameworks that stem from the field of complex systems have been adapted to answer neuroscientific questions. Certainly, using such tools for neuroscientific questions has been insightful for understanding the brain as a complex system. Nevertheless, they encounter limitations when they are adapted for the purpose of understanding the brain, or perhaps better should be stated that, developing approaches which are closer to the neuroscience side can also be instrumental for approaching the brain as a complex system. In this thesis, after an elaboration on the motivation of this endeavor in Chapter 1, we introduce a set of complementary approaches, with the rationale of exploiting the development in the field of systems neuroscience in order to be close to the neuroscience side of the problem, but also still remain connected to the complex systems perspective. Such complementary approaches can be envisioned through different apertures. In this thesis, we introduce our complementary approaches, through the following apertures: neural data analysis (Chapter 2), neural theories (Chapter 3), and cognition (Chapter 4). In Chapter 2, we argue that multi-scale and cross-scale analysis of neural data is one of the important aspects of the neural data analysis from the complex systems perspective toward the brain. Furthermore, we also elaborate that, investigating the brain across scales, is not only important from the abstract perspective of complex systems, but also motivating based on a variety of empirical evidence on coupling between brain activity at different scales, neural coordination and theoretical speculations on neural computation. Based on this motivation we first very briefly discuss some of the relevant cross-scale neural data analysis methodologies and then introduce two novel methodologies that have been developed as parts of this thesis. For micro-Meso relationship we introduced a multi-variate methodology for investigating spike-LFP relationship and in we introduced a methodology for detecting cooperative neural activities (neural events) in local field potentials, that can be used as a trigger to investigate simultaneous activity in larger and smaller scales. A prominent example of these neural events are sharp wave-ripples that has been shown to co-occur with precise coordination in the spiking activity of individual neurons and the large-scale brain activity as well. In Chapter 3, we introduce a new aperture through neural theories. One way of approaching the brain as a complex system is seeking for connections between theoretical frameworks that stem from the field of complex systems and the ones established in neuroscience. On the complex systems side, we consider the criticality hypothesis of the brain that has strong roots in the field of complex systems, and on the neuroscience side, we consider the efficient coding which is one of the most important theoretical frameworks in systems neuroscience. We first briefly introduce the background on efficient coding and criticality, and elaborate further on the motivation behind our integrative approach. We present our interim results, which suggests the two influential, and previously disparate fields – efficient coding, and criticality – might be intimately related. We observed that, in the vicinity of the parameters that leads to optimized performance of a network implementing neural coding, the distribution of avalanche sizes follow a power-law distribution. In we also provide an extensive discussion on the implication of our interim results and its future extensions. Moreover, in we also introduce another perspective which motivates such investigations, namely seeking for potential bridges between neural computation and neural dynamics. In Chapter 4, we argue that binocular rivalry, as a key phenomenon to investigate consciousness, is particularly relevant for a complex systems perspective toward the brain. Based on this insight, we suggest and conduct novel experimental work, namely, studying this phenomenon at a mesoscopic scale, that has not been done before. Surprisingly, in the last 30 years, almost all the previous studies on binocular rivalry were either focused on micro-scale (level of an individual neuron) or the macro-scale (level of the whole brain). Therefore, our work in this domain not only is valuable from the perspective of complex systems, but also for understanding the neural correlate of visual awareness per se. We elaborate on the outcome of this investigation. and were prerequisite for the binocular rivalry experiments. In we elaborate on the importance of studying prefrontal cortex (PFC) (which was the region of interest in our investigation) for understating the neural correlate of visual awareness. In we investigate the basic aspects of neural responses (tuning curves and noise correlations) of PFC units to simple visual stimulation (in a similar setting used for our binocular rivalry experiments). In and we investigate the neural correlate of visual aware- ness at a mesocopic scale (which is motivating from the complex system perspective toward the brain). We show that content of visual awareness is decodable from the population activity of PFC neurons and show oscillatory dynamics of PFC (as a reflection of collective neural activity) can be a relevant signature for perceptual switches. I believe that this is just the very first step toward establishing a connection from a complex systems perspective to cognition and behavior. Various theoretical and experimental steps need to be taken in the future studies to build a solid bridge between cognition and complex systems perspective toward the brain. The last chapter, Chapter 13, is dedicated to an outlook, a subjective perspective on how this research line can be proceeded. In the spirit of this thesis which is searching for principles, I believe we are missing an important aspect of the brain which is its adaptivity. At the end, brain, even the most “complex system”, needs to survive in the environment. Indeed, in the field of complex adaptive systems, the intention is understanding very similar questions in the nature. Inspired by ideas discussed in the field of complex adaptive systems, I introduce a set of new research directions which intend to incorporate the adaptivity aspect of the brain as one of the principles. These research directions also remain close to the neuroscience side, similar to the intention of the research presented in this thesis.

Published
2022

15. Coupling of synaptic inputs to local cortical activity differs among neurons and adapts after stimulus onset.

Author

Wright, Nathaniel C., Hoseini, Mahmood S., Yasar, Tansel Baran, and Wessel, Ralf

Subjects
*NEURONS, *PYRAMIDAL neurons, *TURTLES
Abstract

Cortical activity contributes significantly to the high variability of sensory responses of interconnected pyramidal neurons, which has crucial implications for sensory coding. Yet, largely because of technical limitations of in vivo intracellular recordings, the coupling of a pyramidal neuron's synaptic inputs to the local cortical activity has evaded full understanding. Here we obtained excitatory synaptic conductance (g) measurements from putative pyramidal neurons and local field potential (LFP) recordings from adjacent cortical circuits during visual processing in the turtle whole brain ex vivo preparation. We found a range of g-LFP coupling across neurons. Importantly, for a given neuron, g-LFP coupling increased at stimulus onset and then relaxed toward intermediate values during continued visual stimulation. A model network with clustered connectivity and synaptic depression reproduced both the diversity and the dynamics of g-LFP coupling. In conclusion, these results establish a rich dependence of single-neuron responses on anatomical, synaptic, and emergent network properties. [ABSTRACT FROM AUTHOR]

Published
2017
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16. Adaptation modulates correlated subthreshold response variability in visual cortex.

Author

Wright, Nathaniel C., Hoseini, Mahmood S., and Wessel, Ralf

Abstract

Cortical sensory responses are highly variable across stimulus presentations. This variability can be correlated across neurons (due to some combination of dense intracortical connectivity, cortical activity level, and cortical state), with fundamental implications for population coding. Yet the interpretation of correlated response variability (or “noise correlation”) has remained fraught with difficulty, in part because of the restriction to extracellular neuronal spike recordings. Here, we measured response variability and its correlation at the most microscopic level of electrical neural activity, the membrane potential, by obtaining dual whole cell recordings from pairs of cortical pyramidal neurons during visual processing in the turtle whole brain ex vivo preparation. We found that during visual stimulation, correlated variability adapts toward an intermediate level and that this correlation dynamic is likely mediated by intracortical mechanisms. A model network with external inputs, synaptic depression, and structure reproduced the observed dynamics of correlated variability. These results suggest that intracortical adaptation self-organizes cortical circuits toward a balanced regime at which correlated variability is maintained at an intermediate level. NEW & NOTEWORTHY Correlated response variability has profound implications for stimulus encoding, yet our understanding of this phenomenon is based largely on spike data. Here, we investigate the dynamics and mechanisms of membrane potential-correlated variability (CC) in visual cortex with a combined experimental and computational approach. We observe a visually evoked increase in CC, followed by a fast return to baseline. Our results further suggest a link between this observation and the adaptation-mediated dynamics of emergent network phenomena. [ABSTRACT FROM AUTHOR]

Published
2017
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17. Millimeter to 𝛄-Ray Variability of AGN found in the SPTpol 500 degree2 field

Author

Hood, John and South Pole Telescope Collaboration

Subjects
correlated variability, Astrophysics::High Energy Astrophysical Phenomena, South Pole Telescope, Astrophysics::Instrumentation and Methods for Astrophysics, Active Galactic Nuclei, Astrophysics::Cosmology and Extragalactic Astrophysics, Astrophysics::Galaxy Astrophysics
Abstract

Cosmic Microwave Background (CMB) telescopes are now providing an option for daily monitoring of these objects within limited fields of view. While continuous, high-cadence monitoring of active galactic nuclei (AGN) is now common at gamma-ray, optical, and radio frequencies, AGN monitoring in the millimeter (mm) band has mostly been restricted to short campaigns on targeted sources. Here we use the South Pole Telescope (SPT), which was designed to observe the CMB at arcminute and larger angular scales, to monitor AGN. Between 2013 and 2016, the SPTpol instrument (the second-generation mm-wave camera mounted on the SPT) was used to continuously scan a 500 deg2 field, covering the entire field several times per day with detectors sensitive to polarization and radiation in bands centered at 95 and 150 GHz. This data set enables the monitoring of tens of mm-bright AGN on timescales from years to days at high signal to noise (S/N > 10 in a 36-hour coadd). We use SPTpol 150 GHz observations to monitor the variability of the sources in the mm-waveband and directly compare that to other frequencies. In a pilot study, we focus on the source PKS 2326-502, which has extensive, day-timescale monitoring data in gamma-ray, optical, and now mm-wave data between 2013 and 2016. We present a search for evidence of correlated variability between SPTpol (150 GHz), SMARTS (O/IR) and Fermi (gamma-rays) observations. This pilot study is paving the way for AGN monitoring with current and upcoming CMB experiments like SPT-3G, SO and CMB-S4, including multi-wavelength variability studies with upcoming facilities such as VRO-LSST.

Published
2022
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18. Dynamics of excitatory and inhibitory networks are differentially altered by selective attention.

Author

Snyder, Adam C., Morais, Michael J., and Smith, Matthew A.

Subjects
*SELECTIVITY (Psychology), *EXCITATION (Physiology), *VISUAL cortex, *VISUAL perception, *ATTENTION
Abstract

Inhibition and excitation form two fundamental modes of neuronal interaction, yet we understand relatively little about their distinct roles in service of perceptual and cognitive processes. We developed a multidimensional waveform analysis to identify fast-spiking (putative inhibitory) and regular-spiking (putative excitatory) neurons in vivo and used this method to analyze how attention affects these two cell classes in visual area V4 of the extrastriate cortex of rhesus macaques. We found that putative inhibitory neurons had both greater increases in firing rate and decreases in correlated variability with attention compared with putative excitatory neurons. Moreover, the time course of attention effects for putative inhibitory neurons more closely tracked the temporal statistics of target probability in our task. Finally, the session-to-session variability in a behavioral measure of attention covaried with the magnitude of this effect. Together, these results suggest that selective targeting of inhibitory neurons and networks is a critical mechanism for attentional modulation. [ABSTRACT FROM AUTHOR]

Published
2016
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19. Flexible sensory-motor mapping rules manifest in correlated variability of stimulus and action codes across the brain.

Author

van den Brink, Ruud L., Hagena, Keno, Wilming, Niklas, Murphy, Peter R., Büchel, Christian, and Donner, Tobias H.

Subjects
*HUMAN behavior models, *FUNCTIONAL magnetic resonance imaging
Abstract

Humans and non-human primates can flexibly switch between different arbitrary mappings from sensation to action to solve a cognitive task. It has remained unknown how the brain implements such flexible sensory-motor mapping rules. Here, we uncovered a dynamic reconfiguration of task-specific correlated variability between sensory and motor brain regions. Human participants switched between two rules for reporting visual orientation judgments during fMRI recordings. Rule switches were either signaled explicitly or inferred by the participants from ambiguous cues. We used behavioral modeling to reconstruct the time course of their belief about the active rule. In both contexts, the patterns of correlations between ongoing fluctuations in stimulus- and action-selective activity across visual- and action-related brain regions tracked participants' belief about the active rule. The rule-specific correlation patterns broke down around the time of behavioral errors. We conclude that internal beliefs about task state are instantiated in brain-wide, selective patterns of correlated variability. • Humans can flexibly switch between distinct sensory-motor mapping rules • We tracked stimulus- and action-selective fMRI signals across the brain • Patterns of intrinsic correlations between these signals encoded mapping rules • Correlations tracked graded beliefs about the rule and broke down around errors Primates can rapidly switch between different rules that govern the mapping from stimuli to actions. We uncovered a signature of the dynamic reconfiguration of task-specific networks for flexible sensory-motor decisions in the human brain: intrinsic correlations between the population codes for task-relevant stimuli and actions expressed in different brain regions. [ABSTRACT FROM AUTHOR]

Published
2023
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20. Spatial Correlations in Natural Scenes Modulate Response Reliability in Mouse Visual Cortex.

Author

Rikhye, Rajeev V. and Sur, Mriganka

Subjects
*VISUAL cortex physiology, *STATISTICAL correlation, *LABORATORY mice, *ENCODING, *AUDITORY masking, *NEURAL physiology
Abstract

Intrinsic neuronal variability significantly limits information encoding in the primary visual cortex (V1). Certain stimuli can suppress this intertrial variability to increase the reliability of neuronal responses. In particular, responses to natural scenes, which have broadband spatiotemporal statistics, are more reliable than responses to stimuli such as gratings. However, very little is known about which stimulus statistics modulate reliable coding and how this occurs at the neural ensemble level. Here, we sought to elucidate the role that spatial correlations in natural scenes play in reliable coding. We developed a novel noise-masking method to systematically alter spatial correlations in natural movies, without altering their edge structure. Using high-speed two-photon calcium imaging in vivo, we found that responses in mouse V1 were much less reliable at both the single neuron and population level when spatial correlations were removed from the image. This change in reliability was due to a reorganization of between-neuron correlations. Strongly correlated neurons formed ensembles that reliably and accurately encoded visual stimuli, whereas reducing spatial correlations reduced the activation of these ensembles, leading to an unreliable code. Together with an ensemble-specific normalization model, these results suggest that the coordinated activation of specific subsets of neurons underlies the reliable coding of natural scenes. [ABSTRACT FROM AUTHOR]

Published
2015
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21. Numerical Characterization of Support Recovery in Sparse Regression with Correlated Design

Author

Ankit Kumar, Sharmodeep Bhattacharyya, and Kristofer Bouchard

Subjects
Statistics and Probability, FOS: Computer and information sciences, Sparse regression, Statistics & Probability, Model selection, Statistics - Applications, Mathematical Sciences, Methodology (stat.ME), Modeling and Simulation, Information and Computing Sciences, Compressed sensing, Information criteria, Applications (stat.AP), Correlated variability, Statistics - Methodology
Abstract

Sparse regression is frequently employed in diverse scientific settings as a feature selection method. A pervasive aspect of scientific data that hampers both feature selection and estimation is the presence of strong correlations between predictive features. These fundamental issues are often not appreciated by practitioners, and jeapordize conclusions drawn from estimated models. On the other hand, theoretical results on sparsity-inducing regularized regression such as the Lasso have largely addressed conditions for selection consistency via asymptotics, and disregard the problem of model selection, whereby regularization parameters are chosen. In this numerical study, we address these issues through exhaustive characterization of the performance of several regression estimators, coupled with a range of model selection strategies. These estimators and selection criteria were examined across correlated regression problems with varying degrees of signal to noise, distribution of the non-zero model coefficients, and model sparsity. Our results reveal a fundamental tradeoff between false positive and false negative control in all regression estimators and model selection criteria examined. Additionally, we are able to numerically explore a transition point modulated by the signal-to-noise ratio and spectral properties of the design covariance matrix at which the selection accuracy of all considered algorithms degrades. Overall, we find that SCAD coupled with BIC or empirical Bayes model selection performs the best feature selection across the regression problems considered.

Published
2021
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22. The Neural Basis for Combinatorial Coding in a Cortical Population Response.

Author

Osborne, Leslie C., Palmer, Stephanie E., Lisberger, Stephen G., and Bialek, William

Subjects
*NERVOUS system, *NEURONS, *SENSORY neurons, *NEUROPHYSIOLOGY, *SHORT-term memory
Abstract

We have used a combination of theory and experiment to assess how information is represented in a realistic cortical population response, examining how motion direction and timing is encoded in groups of neurons in cortical area MT. Combining data from several single-unit experiments, we constructed model population responses in small time windows and represented the response in each window as a binary vector of 1s or 0s signifying spikes or no spikes from each cell. We found that patterns of spikes and silence across a population of nominally redundant neurons can carry up to twice as much information about visual motion than does population spike count, even when the neurons respond independently to their sensory inputs. This extra information arises by virtue of the broad diversity of firing rate dynamics found in even very similarly tuned groups of MT neurons. Additionally, specific patterns of spiking and silence can carry more information than the sum of their parts (synergy), opening up the possibility for combinatorial coding in cortex. These results also held for populations in which we imposed levels of nonindependence (correlation) comparable to those found in cortical recordings. Our findings suggest that combinatorial codes are advantageous for representing stimulus information on short time scales, even when neurons have no complicated, stimulus-dependent correlation structure. [ABSTRACT FROM AUTHOR]

Published
2008
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23. Central Contributions to Acoustic Variation in Birdsong.

Author

Sober, Samuel J., Wohlgemuth, Melville J., and Brainard, Michael S.

Subjects
*STEREOTYPES, *BIRDSONGS, *ACOUSTIC vases, *BASAL ganglia, *SOUND recording & reproducing, *ANIMAL sounds
Abstract

Birdsong is a learned behavior remarkable for its high degree of stereotypy. Nevertheless, adult birds display substantial rendition-byrendition variation in the structure of individual song elements or "syllables." Previous work suggests that some of this variation is actively generated by the avian basal ganglia circuitry for purposes of motor exploration. However, it is unknown whether and how natural variations in premotor activity drive variations in syllable structure. Here,werecorded from the premotor nucleus robust nucleus of the arcopallium (RA) in Bengalese finches and measured whether neural activity covaried with syllable structure across multiple renditions of individual syllables. We found that variations in premotor activity were significantly correlated with variations in the acoustic features (pitch, amplitude, and spectral entropy) of syllables in approximately a quarter of all cases. In these cases, individual neural recordings predicted 8.5 ± 0.3% (mean ± SE) of the behavioral variation, and in some cases accounted for 25% or more of trial-by-trial variations in acoustic output. The prevalence and strength of neuron- behavior correlations indicate that each acoustic feature is controlled by a large ensemble of neurons that vary their activity in a coordinated manner. Additionally, we found that correlations with pitch (but not other features) were predominantly positive in sign, supporting a model of pitch production based on the anatomy and physiology of the vocal motor apparatus. Collectively, our results indicate that trial-by-trial variations in spectral structure are indeed under central neural control at the level of RA, consistent with the idea that such variation reflects motor exploration. [ABSTRACT FROM AUTHOR]

Published
2008
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24. Neural Measures of Individual Differences in Selecting and Tracking Multiple Moving Objects.

Author

Drew, Trafton and Vogel, Edward K.

Subjects
*ATTENTION, *NEUROPHYSIOLOGY, *INDIVIDUAL differences, *EVOKED potentials (Electrophysiology), *COGNITION
Abstract

Attention can be divided so that multiple objects can be tracked simultaneously as they move among distractors. Although attentional tracking is known to be highly limited, such that most individuals can track only approximately four objects simultaneously, the neurophysiological mechanisms that underlie this capacity limitation have not been established. Here, we provide electrophysiological measures in humans of the initial selection and sustained attention processes that facilitate attentional tracking. Each measure was modulated by the number of objects the subject was tracking and was highly sensitive to each individual's specific tracking capacity. Consequently, these measures provide strong neurophysiological predictors of an individual's attentional tracking capacity. Moreover, by manipulating the difficulty of these two phases of the task, we observe that the limiting factor underlying tracking capacity can flexibly shift between these two attentional mechanisms depending on the requirements of the task. [ABSTRACT FROM AUTHOR]

Published
2008
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25. Brain Connectivity Related to Working Memory Performance.

Author

Hampson, Michelle, Driesen, Naomi R., Skudlarski, Pawel, Gore, John C., and Constable, R. Todd

Subjects
*SHORT-term memory, *COGNITIVE ability, *MEMORY, *CEREBRAL cortex, *NEUROSCIENCES
Abstract

Several brain areas show signal decreases during many different cognitive tasks in functional imaging studies, including the posterior cingulate cortex (PCC) and a medial frontal region incorporating portions of the medial frontal gyrus and ventral anterior cingulate cortex (MFG/vACC). It has been suggested that these areas are components in a default mode network that is engaged during rest and disengaged during cognitive tasks. This study investigated the functional connectivity between the PCC and MFG/vACC during a working memory task and at rest by examining temporal correlations in magnetic resonance signal levels between the regions. The two regions were functionally connected in both conditions. In addition, performance on the working memory task was positively correlated with the strength of this functional connection not only during the working memory task, but also at rest. Thus, it appears these regions are components of a network that may facilitate or monitor cognitive performance, rather than becoming disengaged during cognitive tasks. In addition, these data raise the possibility that the individual differences in coupling strength between these two regions at rest predict differences in cognitive abilities important for this working memory task. [ABSTRACT FROM AUTHOR]

Published
2006
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26. The Structure of Multi-Neuron Firing Patterns in Primate Retina.

Author

Shlens, Jonathon, Field, Greg D., Gauthier, Jeffrey L., Grivich, Matthew I., Petrusca, Dumitru, Sher, Alexander, Litke, Alan M., and Chichilnisky, E. J.

Subjects
*NEURAL circuitry, *CELLS, *ELECTRODES, *MACAQUES, *RETINA, *NEURONS, *STATISTICAL mechanics
Abstract

Current understanding of many neural circuits is limited by our ability to explore the vast number of potential interactions between different cells. We present a new approach that dramatically reduces the complexity of this problem. Large-scale multi-electrode recordings were used to measure electrical activity in nearly complete, regularly spaced mosaics of several hundred ON and OFF parasol retinal ganglion cells in macaque monkey retina. Parasol cells exhibited substantial pairwise correlations, as has been observed in other species, indicating functional connectivity. However, pairwise measurements alone are insufficient to determine the prevalence of multi-neuron firing patterns, which would be predicted from widely diverging common inputs and have been hypothesized to convey distinct visual messages to the brain. The number of possible multi-neuron firing patterns is far too large to study exhaustively, but this problem may be circumvented if two simple rules of connectivity can be established: (1) multi-cell firing patterns arise from multiple pairwise interactions, and (2) interactions are limited to adjacent cells in the mosaic. Using maximum entropy methods from statistical mechanics, we show that pairwise and adjacent interactions accurately accounted for the structure and prevalence of multi-neuron firing patterns, explaining ∼98% of the departures from statistical independence in parasol cells and ∼99% of the departures that were reproducible in repeated measurements. This approach provides a way to define limits on the complexity of network interactions and thus may be relevant for probing the function of many neural circuits. [ABSTRACT FROM AUTHOR]

Published
2006
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27. Neural Correlates of Fine Depth Discrimination in Monkey Inferior Temporal Cortex.

Author

Uka, Takanori, Tanabe, Seiji, Watanabe, Masayuki, and Fujita, Ichiro

Subjects
*VISION, *DEPTH perception, *VISUAL cortex, *NEURONS, *LABORATORY monkeys
Abstract

Binocular disparity is an important visual cue that gives rise to the perception of depth. Disparity signals are widely spread across the visual cortex, but their relative role is poorly understood. Here, we addressed the correlation between the responses of disparity-selective neurons in the occipitotemporal (ventral) visual pathway and the behavioral discrimination of stereoscopic depth. We recorded activity of disparity-selective neurons in the inferior temporal cortex (IT) while monkeys were engaged in a fine stereoscopic depth discrimination (stereoacuity) task. We found that trial-to-trial fluctuations in neuronal responses correlated with the monkey's perceptual choice. We suggest that disparity signals in the IT, located in the ventral visual pathway, are functionally linked to the discrimination of fine-grain depth. [ABSTRACT FROM AUTHOR]

Published
2005
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29. A search for spectral hysteresis and energy-dependent time lags from x-ray and TeV gamma-ray observations of Mrk 421

Author

Barrio Uña, Juan Abel, Bonnefoy, Simon Francois Albert, Contreras González, José Luis, Domínguez, A., Fidalgo, D., Fonseca González, Mª Victoria, López, M., Rosillo Nievas, Mirella, otros, ..., Barrio Uña, Juan Abel, Bonnefoy, Simon Francois Albert, Contreras González, José Luis, Domínguez, A., Fidalgo, D., Fonseca González, Mª Victoria, López, M., Rosillo Nievas, Mirella, and otros, ...

Abstract

© 2016. The American Astronomical Society. Artículo firmado por más de 10 autores. VERITAS is supported by grants from the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, and by NSERC in Canada. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the instrument.; The MAGIC collaboration would like to thank the Instituto de Astrofisica de Canarias for the excellent working conditions at the Observatorio del Roque de los Muchachos in La Palma. The financial support of the German BMBF and MPG, the Italian INFN and INAF, the Swiss National Fund SNF, the he ERDF under the Spanish MINECO (FPA2015-69818-P, FPA2012-36668, FPA2015-68278-P, FPA2015-69210-C6-2-R, FPA2015-69210-C6-4-R, FPA2015-69210-C6-6-R, AYA2013-47447-C3-1-P, AYA2015-71042-P, ESP2015-71662-C2-2-P, CSD2009-00064), and the Japanese JSPS and MEXT is gratefully acknowledged. This work was also supported by the Spanish Centro de Excelencia "Severo Ochoa" SEV-2012-0234 and SEV-2015-0548, and Unidad de Excelencia "Maria de Maeztu" MDM-2014-0369, by grant 268740 of the Academy of Finland, by the Croatian Science Foundation (HrZZ) Project 09/176 and the University of Rijeka Project 13.12.1.3.02, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0.; This work used data from the Fermi-LAT archive and from the Steward Observatory spectropolarimetric monitoring project, which is supported by Fermi Guest Investigator grants NNX12AO93G and NNX15AU81G funded by NASA.; The OVRO 40-m monitoring program is supported in part by NASA grants NNX08AW31G and NNX11A043G, and NSF grants AST-0808050 and AST-1109911.; M. B. acknowledges support from the International Fulbright Science and Technology Award, and NASA Headquarters under the NASA Earth and Space Science Fellowsh, Blazars are variable emitters across all wavelengths over a wide range of timescales, from months down to minutes. It is therefore essential to observe blazars simultaneously at different wavelengths, especially in the X-ray and gamma-ray bands, where the broadband spectral energy distributions usually peak. In this work, we report on three " target-of-opportunity" observations of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring event at TeV energies in 2014. These observations feature long, continuous, and simultaneous exposures with XMM-Newton (covering the X-ray and optical/ultraviolet bands) and VERITAS (covering the TeV gamma-ray band), along with contemporaneous observations from other gamma-ray facilities (MAGIC and Fermi-Large Area Telescope) and a number of radio and optical facilities. Although neither rapid flares nor significant X-ray/TeV correlation are detected, these observations reveal subtle changes in the X-ray spectrum of the source over the course of a few days. We search the simultaneous X-ray and TeV data for spectral hysteresis patterns and time delays, which could provide insight into the emission mechanisms and the source properties (e. g., the radius of the emitting region, the strength of the magnetic field, and related timescales). The observed broadband spectra are consistent with a one-zone synchrotron self-Compton model. We find that the power spectral density distribution at greater than or similar to 4 x 10(-4) Hz from the X-ray data can be described by a power-law model with an index value between 1.2 and 1.8, and do not find evidence for a steepening of the power spectral index (often associated with a characteristic length scale) compared to the previously reported values at lower frequencies., Ministerio de Economía y Competitividad (MINECO), German BMBF, German MPG, Italian INAF, Italian INFN, Swiss National Fund SNF, Spanish Centro de Excelencia "Severo Ochoa", Unidad de Excelencia "Maria de Maeztu", Academy of Finland, Croatian Science Foundation (HrZZ) Project, University of Rijeka Project, DFG Collaborative Research Centers, Polish MNiSzW, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

Published
2017

30. Multiwavelength study of quiescent states of MRK 421 with unprecedented hard x-ray coverage provided by NuSTAR in 2013

Author

Baloković, M., Paneque, D., Madejski, G., Furniss, A., Chiang, J., the NuSTAR team, Ajello, M., Alexander, D. M., Barret, D., Blandford, R., Boggs, S. E., Christensen, F. E., Craig, W. W., Forster, K., Giommi, P., Grefenstette, B. W., Hailey, C. J., Harrison, F. A., Hornstrup, A., Kitaguchi, T., Koglin, J. E., Madsen, K. K., Mao, P. H., Miyasaka, H., Mori, K., Perri, M., Pivovaroff, M. J., Simonetta Puccetti, Rana, V., Stern, D., Tagliaferri, G., Urry, C. M., Westergaard, N. J., Zhang, W. W., Zoglauer, A., Veritas, The Collaboration, Archambault, S., Archer, A. A., Barnacka, A., Benbow, W., Bird, R., Buckley, J., Bugaev, V., Cerruti, M., Chen, X., Ciupik, L., Connolly, M. P., Cui, W., Dickinson, H. J., Dumm, J., Eisch, J. D., Falcone, A., Feng, Q., Finley, J. P., Fleischhack, H., Fortson, L., Griffin, S., Griffiths, S. T., Grube, J., Gyuk, G., Huetten, M., Haakansson, N., Holder, J., Humensky, T. B., Johnson, C. A., Kaaret, P., Kertzman, M., Khassen, Y., Kieda, D., Krause, M., Krennrich, F., Lang, M. J., Maier, G., Mcarthur, S., Meagher, K., Moriarty, P., Nelson, T., Nieto, D., Ong, R. A., Park, N., Pohl, M., Popkow, A., Pueschel, E., Reynolds, P. T., Richards, G. T., Roache, E., Santander, M., Sembroski, G. H., Shahinyan, K., Smith, A. W., Staszak, D., Telezhinsky, I., Todd, N. W., Tucci, J. V., Tyler, J., Vincent, S., Weinstein, A., Wilhelm, A., Williams, D. A., Zitzer, B., Magic, The Collaboration, Ahnen, M. L., Ansoldi, S., Antonelli, L. A., Antoranz, P., Babic, A., Banerjee, B., Bangale, P., Barres Almeida, U., Barrio, J., Becerra González, J., Bednarek, W., Bernardini, E., Biasuzzi, B., Biland, A., Blanch, O., Bonnefoy, S., Bonnoli, G., Borracci, F., Bretz, T., Carmona, E., Carosi, A., Chatterjee, A., Clavero, R., Colin, P., Colombo, E., Contreras, J. L., Cortina, J., Covino, S., Da Vela, P., Dazzi, F., Angelis, A., Lotto, B., Oña Wilhelmi, E. D., Delgado Mendez, C., Di Pierro, F., Dominis Prester, D., Dorner, D., Doro, M., Einecke, S., Elsaesser, D., Fernández-Barral, A., Fidalgo, D., Fonseca, M. V., Font, L., Frantzen, K., Fruck, C., Galindo, D., García López, R. J., Garczarczyk, M., Garrido Terrats, D., Gaug, M., Giammaria, P., Eisenacher, D., Godinović, N., González Muñoz, A., Guberman, D., Hahn, A., Hanabata, Y., Hayashida, M., Herrera, J., Hose, J., Hrupec, D., Hughes, G., Idec, W., Kodani, K., Konno, Y., Kubo, H., Kushida, J., La Barbera, A., Lelas, D., Lindfors, E., Lombardi, S., Longo, F., López, M., López-Coto, R., López-Oramas, A., Lorenz, E., Majumdar, P., Makariev, M., Mallot, K., Maneva, G., Manganaro, M., Mannheim, K., Maraschi, L., Marcote, B., Mariotti, M., Martínez, M., Mazin, D., Menzel, U., Miranda, J. M., Mirzoyan, R., Moralejo, A., Moretti, E., Nakajima, D., Neustroev, V., Niedzwiecki, A., Nievas-Rosillo, M., Nilsson, K., Nishijima, K., Noda, K., Orito, R., Overkemping, A., Paiano, S., Palacio, S., Palatiello, M., Paoletti, R., Paredes, J. M., Paredes-Fortuny, X., Persic, M., Poutanen, J., Prada Moroni, P. G., Prandini, E., Puljak, I., Rhode, W., Ribó, M., Rico, J., Rodriguez Garcia, J., Saito, T., Satalecka, K., Scapin, V., Schultz, C., Schweizer, T., Shore, S. N., Sillanpää, A., Sitarek, J., Snidaric, I., Sobczynska, D., Stamerra, A., Steinbring, T., Strzys, M., Takalo, L. O., Takami, H., Tavecchio, F., Temnikov, P., Terzić, T., Tescaro, D., Teshima, M., Thaele, J., Torres, D. F., Toyama, T., Treves, A., Verguilov, V., Vovk, I., Ward, J. E., Will, M., Wu, M. H., Zanin, R., Collaborators, External, Perkins, J., Verrecchia, F., Leto, C., Böttcher, M., Villata, M., Raiteri, C. M., Acosta-Pulido, J. A., Bachev, R., Berdyugin, A., Blinov, D. A., Carnerero, M. I., Chen, W. P., Chinchilla, P., Damljanovic, G., Eswaraiah, C., Grishina, T. S., Ibryamov, S., Jordan, B., Jorstad, S. G., Joshi, M., Kopatskaya, E. N., Kurtanidze, O. M., Kurtanidze, S. O., Larionova, E. G., Larionova, L. V., Larionov, V. M., Latev, G., Lin, H. C., Marscher, A. P., Mokrushina, A. A., Morozova, D. A., Nikolashvili, M. G., Semkov, E., Strigachev, A., Troitskaya, Yu V., Troitsky, I. S., Vince, O., Barnes, J., Güver, T., Moody, J. W., Sadun, A. C., Sun, S., Hovatta, T., Richards, J. L., Max-Moerbeck, W., Readhead, A. C., Lähteenmäki, A., Tornikoski, M., Tammi, J., Ramakrishnan, V., Reinthal, R., Angelakis, E., Fuhrmann, L., Myserlis, I., Karamanavis, V., Sievers, A., Ungerechts, H., Zensus, J. A., Universitat de Barcelona, Department of Radio Science and Engineering, Aalto-yliopisto, Aalto University, Glawion , D., ~, Baloković, M., Paneque, D., Madejski, G., Furniss, A., Chiang, J., Ajello, M., Alexander, D. M., Barret, D., Blandford, R. D., Boggs, S. E., Christensen, F. E., Craig, W. W., Forster, K., Giommi, P., Grefenstette, B., Hailey, C., Harrison, F. A., Hornstrup, A., Kitaguchi, T., Koglin, J. E., Madsen, K. K., Mao, P. H., Miyasaka, H., Mori, K., Perri, M., Pivovaroff, M. J., Puccetti, S., Rana, V., Stern, D., Tagliaferri, G., Urry, C. M., Westergaard, N. J., Zhang, W. W., Zoglauer, A., Archambault, S., Archer, A., Barnacka, A., Benbow, W., Bird, R., Buckley, J. H., Bugaev, V., Cerruti, M., Chen, X., Ciupik, L., Connolly, M. P., Cui, W., Dickinson, H. J., Dumm, J., Eisch, J. D., Falcone, A., Feng, Q., Finley, J. P., Fleischhack, H., Fortson, L., Griffin, S., Griffiths, S. T., Grube, J., Gyuk, G., Huetten, M., Håkansson, N., Holder, J., Humensky, T. B., Johnson, C. A., Kaaret, P., Kertzman, M., Khassen, Y., Kieda, D., Krause, M., Krennrich, F., Lang, M. J., Maier, G., Mcarthur, S., Meagher, K., Moriarty, P., Nelson, T., Nieto, D., Ong, R. A., Park, N., Pohl, M., Popkow, A., Pueschel, E., Reynolds, P. T., Richards, G. T., Roache, E., Santander, M., Sembroski, G. H., Shahinyan, K., Smith, A. W., Staszak, D., Telezhinsky, I., Todd, N. W., Tucci, J. V., Tyler, J., Vincent, S., Weinstein, A., Wilhelm, A., Williams, D. A., Zitzer, B., Ahnen, M. L., Ansoldi, S., Antonelli, L. A., Antoranz, P., Babic, A., Banerjee, B., Bangale, P., De Almeida, U. Barre, Barrio, J. A., González, J. Becerra, Bednarek, W., Bernardini, E., Biasuzzi, B., Biland, A., Blanch, O., Bonnefoy, S., Bonnoli, G., Borracci, F., Bretz, T., Carmona, E., Carosi, A., Chatterjee, A., Clavero, R., Colin, P., Colombo, E., Contreras, J. L., Cortina, J., Covino, S., Da Vela, P., Dazzi, F., De Angelis, A., De Lotto, B., De Ońa Wilhelmi, E., Mendez, C. Delgado, Di Pierro, F., Prester, D. Domini, Dorner, D., Doro, M., Einecke, S., Elsaesser, D., Fernández Barral, A., Fidalgo, D., Fonseca, M. V., Font, L., Frantzen, K., Fruck, C., Galindo, D., López, R. J. García, Garczarczyk, M., Terrats, D. Garrido, Gaug, M., Giammaria, P., Glawion, D., Godinović, N., González Muñoz, A., Guberman, D., Hahn, A., Hanabata, Y., Hayashida, M., Herrera, J., Hose, J., Hrupec, D., Hughes, G., Idec, W., Kodani, K., Konno, Y., Kubo, H., Kushida, J., La Barbera, A., Lelas, D., Lindfors, E., Lombardi, S., Longo, Francesco, López, M., López Coto, R., López Oramas, A., Lorenz, E., Majumdar, P., Makariev, M., Mallot, K., Maneva, G., Manganaro, M., Mannheim, K., Maraschi, L., Marcote, B., Mariotti, M., Martínez, M., Mazin, D., Menzel, U., Miranda, J. M., Mirzoyan, R., Moralejo, A., Moretti, E., Nakajima, D., Neustroev, V., Niedzwiecki, A., Rosillo, M. Nieva, Nilsson, K., Nishijima, K., Noda, K., Orito, R., Overkemping, A., Paiano, S., Palacio, J., Palatiello, Michele, Paoletti, R., Paredes, J. M., Paredes Fortuny, X., Persic, M., Poutanen, J., Moroni, P. G. Prada, Prandini, E., Puljak, I., Rhode, W., Ribó, M., Rico, J., Garcia, J. Rodriguez, Saito, T., Satalecka, K., Scapin, V., Schultz, C., Schweizer, T., Shore, S. N., Sillanpää, A., Sitarek, J., Snidaric, I., Sobczynska, D., Stamerra, A., Steinbring, T., Strzys, M., Takalo, L., Takami, H., Tavecchio, F., Temnikov, P., Terzić, T., Tescaro, D., Teshima, M., Thaele, J., Torres, D. F., Toyama, T., Treves, A., Verguilov, V., Vovk, I., Ward, J. E., Will, M., Wu, M. H., Zanin, R., Perkins, J., Verrecchia, F., Leto, C., Böttcher, M., Villata, M., Raiteri, C. M., Acosta Pulido, J. A., Bachev, R., Berdyugin, A., Blinov, D. A., Carnerero, M. I., Chen, W. P., Chinchilla, P., Damljanovic, G., Eswaraiah, C., Grishina, T. S., Ibryamov, S., Jordan, B., Jorstad, S. G., Joshi, M., Kopatskaya, E. N., Kurtanidze, O. M., Kurtanidze, S. O., Larionova, E. G., Larionova, L. V., Larionov, V. M., Latev, G., Lin, H. C., Marscher, A. P., Mokrushina, A. A., Morozova, D. A., Nikolashvili, M. G., Semkov, E., Smith, P. S., Strigachev, A., Troitskaya, Y. u. V., Troitsky, I. S., Vince, O., Barnes, J., Güver, T., Moody, J. W., Sadun, A. C., Sun, S., Hovatta, T., Richards, J. L., Max Moerbeck, W., Readhead, A. C. R., Lähteenmäki, A., Tornikoski, M., Tammi, J., Ramakrishnan, V., Reinthal, R., Angelakis, E., Fuhrmann, L., Myserlis, I., Karamanavis, V., Sievers, A., Ungerechts, H., and Zensus, J. A.

Subjects
galaxie [X-rays], Photon, PARTICLE-ACCELERATION, ELECTRON ACCELERATION, BACKGROUND-RADIATION, Astrophysics, Electron, Nonthermal, BL Lacertae objects: individual (Markarian 421), galaxies: active, gamma rays: general, radiation mechanisms: nonthermal, X-rays: galaxies, 01 natural sciences, individual: Markarian 421 [BL Lacertae objects], LARGE-AREA TELESCOPE, individual (Markarian 421) [BL Lacertae objects], law.invention, Raigs gamma, law, MAGNETIC RECONNECTION, 010303 astronomy & astrophysics, Physics, High Energy Astrophysical Phenomena (astro-ph.HE), Large area telescope, TEV BLAZARS, gamma, rays: general, ACTIVE GALACTIC NUCLEI, BL LACERTAE OBJECTS, LAC OBJECTS, CORRELATED VARIABILITY, Synchrotron, galaxies [X-rays], Spectral energy distribution, Degree of polarization, Electrónica, Física nuclear, Raigs X, Electricidad, Astrophysics - High Energy Astrophysical Phenomena, BL Lac object, Astronomy and Astrophysics, Space and Planetary Science, active [Galaxies], Astrophysics::High Energy Astrophysical Phenomena, Particle acceleration, general [Gamma rays], FOS: Physical sciences, Photon energy, 0103 physical sciences, X-rays, Radiation mechanisms, BL Lacertae objects, individual, Markarian 421, galaxies, active, gamma rays, general, radiation mechanisms, nonthermal, 010308 nuclear & particles physics, Gamma rays, Astronomy and Astrophysic, nonthermal [Radiation mechanisms], Light curve, Galaxies, Galàxies, Background radiation, ddc:520
Abstract

The astrophysical journal 819(2), 156 (2016). doi:10.3847/0004-637X/819/2/156, We present coordinated multiwavelength observations of the bright, nearby BL Lacertae object Mrk 421 taken in 2013 January–March, involving GASP-WEBT, Swift, NuSTAR, Fermi-LAT, MAGIC, VERITAS, and other collaborations and instruments, providing data from radio to very high energy (VHE) γ-ray bands. NuSTAR yielded previously unattainable sensitivity in the 3–79 keV range, revealing that the spectrum softens when the source is dimmer until the X-ray spectral shape saturates into a steep ${\rm{\Gamma }}\approx 3$ power law, with no evidence for an exponential cutoff or additional hard components up to ~80 keV. For the first time, we observed both the synchrotron and the inverse-Compton peaks of the spectral energy distribution (SED) simultaneously shifted to frequencies below the typical quiescent state by an order of magnitude. The fractional variability as a function of photon energy shows a double-bump structure that relates to the two bumps of the broadband SED. In each bump, the variability increases with energy, which, in the framework of the synchrotron self-Compton model, implies that the electrons with higher energies are more variable. The measured multi band variability, the significant X-ray-to-VHE correlation down to some of the lowest fluxes ever observed in both bands, the lack of correlation between optical/UV and X-ray flux, the low degree of polarization and its significant (random) variations, the short estimated electron cooling time, and the significantly longer variability timescale observed in the NuSTAR light curves point toward in situ electron acceleration and suggest that there are multiple compact regions contributing to the broadband emission of Mrk 421 during low-activity states., Published by Univ.11032, Chicago, Ill. [u.a.]

Published
2016
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31. A search for spectral hysteresis and energy-dependent time lags from X-ray and TeV gamma-ray observations of Mrk 421

Author

Abeysekara, A. U., Archambault, S., Chen, X., De Lotto, B., Wilhelmi, E. de Oña, Di Pierro, F., Doert, M., Domínguez, A., Prester, D. Dominis, Dorner, D., Doro, M., Einecke, S., Glawion, D. Eisenacher, Ciupik, L., Elsaesser, D., Engelkemeier, M., Ramazani, V. Fallah, Fernández-Barral, A., Fidalgo, D., Fonseca, M. V., Font, L., Fruck, C., Galindo, D., López, R. J. García, Connolly, M. P., Garczarczyk, M., Gaug, M., Giammaria, P., Godinović, N., Gora, D., Guberman, D., Hadasch, D., Hahn, A., Hassan, T., Hayashida, M., Cui, W., Herrera, J., Hose, J., Hrupec, D., Hughes, G., Idec, W., Kodani, K., Konno, Y., Kubo, H., Kushida, J., Lelas, D., Eisch, J. D., Lindfors, E., Lombardi, S., Longo, F., López, M., López-Coto, R., Majumdar, P., Makariev, M., Mallot, K., Maneva, G., Manganaro, M., Falcone, A., Mannheim, K., Maraschi, L., Marcote, B., Mariotti, M., Martínez, M., Mazin, D., Menzel, U., Mirzoyan, R., Moralejo, A., Moretti, E., Feng, Q., Nakajima, D., Neustroev, V., Niedzwiecki, A., Rosillo, M. Nievas, Nilsson, K., Nishijima, K., Noda, K., Nogués, L., Nöthe, M., Paiano, S., Finley, J. P., Palacio, J., Palatiello, M., Paneque, D., Paoletti, R., Paredes, J. M., Paredes-Fortuny, X., Pedaletti, G., Peresano, M., Perri, L., Persic, M., Fleischhack, H., Poutanen, J., Moroni, P. G. Prada, Prandini, E., Puljak, I., Garcia, J. R., Reichardt, I., Rhode, W., Ribó, M., Rico, J., Saito, T., Flinders, A., Satalecka, K., Schroeder, S., Schweizer, T., Shore, S. N., Sillanpää, A., Sitarek, J., Snidaric, I., Sobczynska, D., Stamerra, A., Strzys, M., Archer, A., Fortson, L., Surić, T., Takalo, L., Tavecchio, F., Temnikov, P., Terzić, T., Tescaro, D., Teshima, M., Torres, D. F., Torres-Albà, N., Toyama, T., Furniss, A., Treves, A., Vanzo, G., Acosta, M. Vazquez, Vovk, I., Ward, J. E., Will, M., Wu, M. H., Zanin, R., Hovatta, T., Perez, I. de la Calle, Griffin, S., Smith, P. S., Racero, E., Baloković, M., VERITAS Collaboration, MAGIC Collaboration, Hütten, M., Håkansson, N., Hanna, D., Hervet, O., Holder, J., Humensky, T. B., Kaaret, P., Benbow, W., Kar, P., Kertzman, M., Kieda, D., Krause, Maria, Kumar, S., Lang, M. J., Maier, G., McArthur, S., McCann, A., Meagher, K., Bird, R., Moriarty, P., Mukherjee, R., Nieto, D., O'Brien, S., Ong, R. A., Otte, A. N., Park, N., Pelassa, V., Pohl, Martin, Popkow, A., Buchovecky, M., Pueschel, E., Ragan, K., Reynolds, P. T., Richards, G. T., Roache, E., Sadeh, I., Santander, M., Sembroski, G. H., Shahinyan, K., Staszak, D., Buckley, J. H., Telezhinsky, I., Tucci, J. V., Tyler, J., Wakely, S. P., Weinstein, A., Wilhelm, Alina, Williams, D. A., Ahnen, M. L., Ansoldi, S., Antonelli, L. A., Bugaev, V., Antoranz, P., Arcaro, C., Babic, A., Banerjee, B., Bangale, P., de Almeida, U. Barres, Barrio, J. A., González, J. Becerra, Bednarek, W., Bernardini, E., Cardenzana, J. V, Berti, A., Biasuzzi, B., Biland, A., Blanch, O., Bonnefoy, S., Bonnoli, G., Borracci, F., Bretz, T., Carosi, R., Carosi, A., Cerruti, M., Chatterjee, A., Colin, P., Colombo, E., Contreras, J. L., Cortina, J., Covino, S., Cumani, P., Da Vela, P., Dazzi, F., De Angelis, A., Department of Radio Science and Engineering, Aalto-yliopisto, Aalto University, Universitat de Barcelona, Abeysekara, A. U., Archambault, S., Archer, A., Benbow, W., Bird, R., Buchovecky, M., Buckley, J. H., Bugaev, V., Cardenzana, J. V., Cerruti, M., Chen, X., Ciupik, L., Connolly, M. P., Cui, W., Eisch, J. D., Falcone, A., Feng, Q., Finley, J. P., Fleischhack, H., Flinders, A., Fortson, L., Furniss, A., Griffin, S., Hakansson, M. H. N., Hanna, D., Hervet, O., Holder, J., Humensky, T. B., Kaaret, P., Kar, P., Kertzman, M., Kieda, D., Krause, M., Kumar, S., Lang, M. J., Maier, G., Mcarthur, S., Mccann, A., Meagher, K., Moriarty, P., Mukherjee, R., Nieto, D., Ong, S. O. R. A., Otte, A. N., Park, N., Pelassa, V., Pohl, M., Popkow, A., Pueschel, E., Ragan, K., Reynolds, P. T., Richards, G. T., Roache, E., Sadeh, I., Santander, M., Sembroski, G. H., Shahinyan, K., Staszak, D., Telezhinsky, I., Tucci, J. V., Tyler, J., Wakely, S. P., Weinstein, A., Wilhelm, A., Williams, D. A., Ahnen, M. L., Ansoldi, S., Antonelli, L. A., Antoranz, P., Arcaro, C., Babic, A., Banerjee, B., Bangale, P., Almeida, U. Barres De, Barrio, J. A., González, J. Becerra, Bednarek, W., Bernardini, E., Berti, Alessio, Biasuzzi, B., Biland, A., Blanch, O., Bonnefoy, S., Bonnoli, G., Borracci, F., Bretz, T., Carosi, R., Carosi, A., Chatterjee, A., Colin, P., Colombo, E., Contreras, J. L., Cortina, J., Covino, S., Cumani, P., Da Vela, P., Dazzi, F., De Angelis, A., De Lotto, B., De Ona Wilhelmi, E., Di Pierro, F., Doert, M., Domínguez, A., Prester, D. Domini, Dorner, D., Doro, M., Einecke, S., Glawion, D. Eisenacher, Elsaesser, D., Engelkemeier, M., Ramazani, V. Fallah, Fernández Barral, A., Fidalgo, D., Fonseca, M. V., Font, L., Fruck, C., Galindo, D., López, R. J. García, Garczarczyk, M., Gaug, M., Giammaria, P., Godinović, N., Gora, D., Guberman, D., Hadasch, D., Hahn, A., Hassan, T., Hayashida, M., Herrera, J., Hose, J., Hrupec, D., Hughes, G., Idec, W., Kodani, K., Konno, Y., Kubo, H., Kushida, J., Lelas, D., Lindfors, E., Lombardi, S., Longo, Francesco, López, M., López Coto, R., Majumdar, P., Makariev, M., Mallot, K., Maneva, G., Manganaro, M., Mannheim, K., Maraschi, L., Marcote, B., Mariotti, M., Martínez, M., Mazin, D., Menzel, U., Mirzoyan, R., Moralejo, A., Moretti, E., Nakajima, D., Neustroev, V., Niedzwiecki, A., Rosillo, M. Nieva, Nilsson, K., Nishijima, K., Noda, K., Nogués, L., Nöthe, M., Paiano, S., Palacio, J., Palatiello, Michele, Paneque, D., Paoletti, R., Paredes, J. M., Paredes Fortuny, X., Pedaletti, G., Peresano, M., Perri, L., Persic, M., Poutanen, J., Moroni, P. G. Prada, Prandini, E., Puljak, I., Garcia, J. R., Reichardt, I., Rhode, W., Ribó, M., Rico, J., Saito, T., Satalecka, K., Schroeder, S., Schweizer, T., Shore, S. N., Sillanp, A., Sitarek, J., Snidaric, I., Sobczynska, D., Stamerra, A., Strzys, M., Surić, T., Takalo, L., Tavecchio, F., Temnikov, P., Terzić, T., Tescaro, D., Teshima, M., Torres, D. F., Torres Albà, N., Toyama, T., Treves, A., Vanzo, G., Acosta, M. Vazquez, Vovk, I., Ward, J. E., Will, M., Wu, M. H., Zanin, R., Hovatta, T., De La Calle Perez, I., Smith, P. S., Racero, E., and Baloković, M.

Subjects
bl-lacertae objects, blazars, Astrophysics, telescope, 01 natural sciences, Spectral line, individual (Markarian 421) [BL Lacertae objects], law.invention, Raigs gamma, law, emission, MAGIC (telescope), 010303 astronomy & astrophysics, High Energy Astrophysical Phenomena (astro-ph.HE), Physics, Spectral index, Gamma ray, Radius, lac objects, non-thermal [radiation mechanisms], Synchrotron, xmm-newton, BL Lacertae objects: individual (Markarian 421), galaxies: active, gamma rays: general, radiation mechanisms: non-thermal, Astronomy and Astrophysics, Space and Planetary Science, correlated variability, active [galaxies], Física nuclear, Astrophysics - High Energy Astrophysical Phenomena, multiwavelength observations, Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, 0103 physical sciences, Blazar, 010308 nuclear & particles physics, Gamma rays, Spectral density, Astronomy and Astrophysic, Galaxies, Galàxies, active galactic nuclei, markarian-421, ddc:520, galaxies: active – BL Lacertae objects: individual: (Markarian 421), general [gamma rays]
Abstract

The astrophysical journal 834(1), 2 (2017). doi:10.3847/1538-4357/834/1/2, Blazars are variable emitters across all wavelengths over a wide range of timescales, from months down to minutes. It is therefore essential to observe blazars simultaneously at different wavelengths, especially in the X-ray and gamma-ray bands, where the broadband spectral energy distributions usually peak. In this work, we report on three "target-of-opportunity" observations of Mrk 421, one of the brightest TeV blazars, triggered by a strong flaring event at TeV energies in 2014. These observations feature long, continuous, and simultaneous exposures with XMM-Newton (covering the X-ray and optical/ultraviolet bands) and VERITAS (covering the TeV gamma-ray band), along with contemporaneous observations from other gamma-ray facilities (MAGIC and Fermi-Large Area Telescope) and a number of radio and optical facilities. Although neither rapid flares nor significant X-ray/TeV correlation are detected, these observations reveal subtle changes in the X-ray spectrum of the source over the course of a few days. We search the simultaneous X-ray and TeV data for spectral hysteresis patterns and time delays, which could provide insight into the emission mechanisms and the source properties (e.g., the radius of the emitting region, the strength of the magnetic field, and related timescales). The observed broadband spectra are consistent with a one-zone synchrotron self-Compton model. We find that the power spectral density distribution at $\gtrsim$ 4 × 10$^{−4}$ Hz from the X-ray data can be described by a power-law model with an index value between 1.2 and 1.8, and do not find evidence for a steepening of the power spectral index (often associated with a characteristic length scale) compared to the previously reported values at lower frequencies., Published by IOP Publ., Chicago, Ill. [u.a.]

Published
2016
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32. Insights into decision making using choice probability

Author

Trinity B. Crapse and Michele A. Basso

Subjects
multiple neuron recording, vision, Observer (quantum physics), Physiology, media_common.quotation_subject, Physiological, Models, Neurological, Population, perception, Choice Behavior, Medical and Health Sciences, Feedback, Models, Perception, sensation, Premovement neuronal activity, Animals, Humans, education, signal detection theory, media_common, Probability, Feedback, Physiological, Systems neuroscience, education.field_of_study, Neurology & Neurosurgery, General Neuroscience, Psychology and Cognitive Sciences, Feed forward, Brain, Neurophysiology, eye movements, nervous system, correlated variability, Metric (mathematics), Neurological, Call for Papers, neurophysiology, Psychology, Neuroscience, Cognitive psychology
Abstract

© 2015 the American Physiological Society. A long-standing question in systems neuroscience is how the activity of single neurons gives rise to our perceptions and actions. Critical insights into this question occurred in the last part of the 20th century when scientists began linking modulations of neuronal activity directly to perceptual behavior. A significant conceptual advance was the application of signal detection theory to both neuronal activity and behavior, providing a quantitative assessment of the relationship between brain and behavior. One metric that emerged from these efforts was choice probability (CP), which provides information about how well an ideal observer can predict the choice an animal makes from a neuron’s discharge rate distribution. In this review, we describe where CP has been studied, locational trends in the values found, and why CP values are typically so low. We discuss its dependence on correlated activity among neurons of a population, assess whether it arises from feedforward or feedback mechanisms, and investigate what CP tells us about how many neurons are required for a decision and how they are pooled to do so.

Published
2015
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33. Cortical activity is more stable when sensory stimuli are consciously perceived

Author

Lionel Naccache, Stanislas Dehaene, Aaron Schurger, Jacobo D. Sitt, and Ioannis Sarigiannidis

Subjects
Adult, Male, medicine.medical_specialty, Visual perception, Time Factors, genetic structures, Consciousness, media_common.quotation_subject, Sensation, Sensory system, Electroencephalography, Stimulus (physiology), Audiology, Developmental psychology, Young Adult, Perception, pattern similarity, Physical Stimulation, Task Performance and Analysis, medicine, Humans, Evoked Potentials, media_common, Cerebral Cortex, Multidisciplinary, medicine.diagnostic_test, Subjective report, Reproducibility of Results, directional variance, dynamical systems, PNAS Plus, correlated variability, Consciousness Disorders, Female, Psychology
Abstract

According to recent evidence, stimulus-tuned neurons in the cerebral cortex exhibit reduced variability in firing rate across trials, after the onset of a stimulus. However, in order for a reduction in variability to be directly relevant to perception and behavior, it must be realized within trial--the pattern of activity must be relatively stable. Stability is characteristic of decision states in recurrent attractor networks, and its possible relevance to conscious perception has been suggested by theorists. However, it is difficult to measure on the within-trial time scales and broadly distributed spatial scales relevant to perception. We recorded simultaneous magneto- and electroencephalography (MEG and EEG) data while subjects observed threshold-level visual stimuli. Pattern-similarity analyses applied to the data from MEG gradiometers uncovered a pronounced decrease in variability across trials after stimulus onset, consistent with previous single-unit data. This was followed by a significant divergence in variability depending upon subjective report (seen/unseen), with seen trials exhibiting less variability. Applying the same analysis across time, within trial, we found that the latter effect coincided in time with a difference in the stability of the pattern of activity. Stability alone could be used to classify data from individual trials as "seen" or "unseen." The same metric applied to EEG data from patients with disorders of consciousness exposed to auditory stimuli diverged parametrically according to clinically diagnosed level of consciousness. Differences in signal strength could not account for these results. Conscious perception may involve the transient stabilization of distributed cortical networks, corresponding to a global brain-scale decision.

Published
2015

34. Very high-energy gamma-ray follow-up program using neutrino triggers from IceCube

Author

Barrio Uña, Juan Abel, Bonnefoy, Simon Francois Albert, Contreras González, José Luis, Domínguez, A., Fidalgo, D., Fonseca González, Mª Victoria, López, M., Nievas Rosillo, Mireia, Barrio Uña, Juan Abel, Bonnefoy, Simon Francois Albert, Contreras González, José Luis, Domínguez, A., Fidalgo, D., Fonseca González, Mª Victoria, López, M., and Nievas Rosillo, Mireia

Abstract

© 2016 IOP Publishing Ltd and Sissa Medialab srl Artículo firmado por más de 10 autores We acknowledge the support from the following agencies: U.S. National Science Foundation-Office of Polar Programs, U.S. National Science Foundation-Physics Division, University of Wisconsin Alumni Research Foundation, the Grid Laboratory Of Wisconsin (GLOW) grid infrastructure at the University of Wisconsin - Madison, the Open Science Grid (OSG) grid infrastructure; U.S. Department of Energy, and National Energy Research Scientific Computing Center, the Louisiana Optical Network Initiative (LONI) grid computing resources; Natural Sciences and Engineering Research Council of Canada, WestGrid and Compute/Calcul Canada; Swedish Research Council, Swedish Polar Research Secretariat, Swedish National Infrastructure for Computing (SNIC), and Knut and Alice Wallenberg Foundation, Sweden; German Ministry for Education and Research (BMBF), Deutsche Forschungsgemeinschaft (DFG), Helmholtz Alliance for Astroparticle Physics (HAP), Research Department of Plasmas with Complex Interactions (Bochum), Germany; Fund for Scientific Research (FNRS-FWO), FWO Odysseus programme, Flanders Institute to encourage scientific and technological research in industry (IWT), Belgian Federal Science Policy Office (Belspo); University of Oxford, United Kingdom; Marsden Fund, NewZealand; Australian Research Council; Japan Society for Promotion of Science (JSPS); the Swiss National Science Foundation (SNSF), Switzerland; National Research Foundation of Korea (NRF); Villum Fonden, Danish National Research Foundation (DNRF), Denmark.; The VERITAS Collaboration acknowledges the support of the U.S. Department of Energy Office of Science, the U.S. National Science Foundation and the Smithsonian Institution, and NSERC in Canada. We also acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions in the construction and operation of the inst, We describe and report the status of a neutrino-triggered program in IceCube that generates real-time alerts for gamma-ray follow-up observations by atmospheric-Cherenkov telescopes (MAGIC and VERITAS). While IceCube is capable of monitoring the whole sky continuously, high-energy gamma-ray telescopes have restricted fields of view and in general are unlikely to be observing a potential neutrino-flaring source at the time such neutrinos are recorded. The use of neutrino-triggered alerts thus aims at increasing the availability of simultaneous multi-messenger data during potential neutrino flaring activity, which can increase the discovery potential and constrain the phenomenological interpretation of the high-energy emission of selected source classes (e. g. blazars). The requirements of a fast and stable online analysis of potential neutrino signals and its operation are presented, along with first results of the program operating between 14 March 2012 and 31 December 2015., Ministerio de Economía y Competitividad (MINECO), Swedish Research Council, Swedish Polar Research Secretariat, Swedish National Infrastructure for Computing (SNIC), Knut and Alice Wallenberg Foundation, Sweden, German Ministry for Education and Research (BMBF), Deutsche Forschungsgemeinschaft (DFG), Helmholtz Alliance for Astroparticle Physics (HAP), Research Department of Plasmas with Complex Interactions (Bochum), Germany, Fund for Scientific Research (FNRS-FWO), FWO Odysseus programme, Flanders Institute to encourage scientific and technological research in industry (IWT), Belgian Federal Science Policy Office (Belspo), University of Oxford, United Kingdom, Swiss National Science Foundation (SNSF), Switzerland, Villum Fonden, Danish National Research Foundation (DNRF), Denmark, German BMBF and MPG, Italian INFN and INAF, Spanish Centro de Excelencia "Severo Ochoa", Unidad de Excelencia "Maria de Maeztu", Academy of Finland, Croatian Science Foundation (HrZZ) Project, University of Rijeka Project, DFG Collaborative Research Centers, Polish MNiSzW, Depto. de Estructura de la Materia, Física Térmica y Electrónica, Fac. de Ciencias Físicas, TRUE, pub

Published
2016

35. Observation of Markarian 421 in TeV gamma rays over a 14-year time span

Author

M. J. Lang, J. Tyler, J. H. Buckley, D. A. Carter-Lewis, E. Collins-Hughes, G. H. Gillanders, S. P. Wakely, Daniel Gall, L. Ciupik, N. Galante, E. Roache, A. N. Otte, H. Krawczynski, T. Reynolds, M. P. Connolly, D. S. Hanna, Ralph Bird, G. Finnegan, Geza Gyuk, K. Ragan, A. W. Smith, C. Duke, A. Pichel, G. H. Sembroski, Lucy Fortson, R. Welsing, Abraham D. Falcone, T. C. Arlen, V. Bugaev, Matthew R. Orr, J. P. Finley, Wei Cui, Y. Khassen, V. A. Acciari, T. Aune, T. C. Weekes, S. M. Bradbury, John Toner, A. C. Rovero, A. Cesarini, A. S. Madhavan, Wystan Benbow, Martin Pohl, John L. Quinn, P. Moriarty, R. A. Ong, K. G. Gibbs, J. Dumm, J. S. Perkins, S. J. Fegan, S. Federici, A. Weinstein, Stephanie Wissel, M. Schroedter, I. de la Calle Perez, A. Varlotta, A. Bouvier, P. Kaaret, J. E. Ward, David Kieda, M. Theiling, Sinéad M. Griffin, M. Kertzman, A. O'Faolain de Bhroithe, D. J. Fegan, T. J. N. Nelson, J. A. Gaidos, David A. Williams, I. Telezhinsky, T. B. Humensky, Julie McEnery, D. Horan, D. Petry, J. Grube, M. Vivier, and F. Krennrich

Subjects
multiwavelength observations, Cosmology and Nongalactic Astrophysics (astro-ph.CO), Active galactic nucleus, Ciencias Físicas, Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, Flux, Astrophysics, telescope, timing explorer, 01 natural sciences, Spectral line, law.invention, purl.org/becyt/ford/1 [https], Telescope, law, blazar markarian-421, emission, 0103 physical sciences, crab-nebula, flare, AGN, Blazar, tev gamma rays, 010303 astronomy & astrophysics, High Energy Astrophysical Phenomena (astro-ph.HE), Physics, Markarian 421, 010308 nuclear & particles physics, markarian 421, Gamma ray, Astronomy, Institut für Physik und Astronomie, Astronomy and Astrophysics, purl.org/becyt/ford/1.3 [https], TeV gamma rays, Astronomía, Crab Nebula, correlated variability, active galactic nuclei, ddc:540, agn, Astrophysics - High Energy Astrophysical Phenomena, Markarian galaxies, CIENCIAS NATURALES Y EXACTAS, energy, Astrophysics - Cosmology and Nongalactic Astrophysics
Abstract

The variability of the blazar Markarian 421 in TeV gamma rays over a 14-year time period has been explored with the Whipple 10 m telescope. It is shown that the dynamic range of its flux variations is large and similar to that in X-rays. A correlation between the X-ray and TeV energy bands is observed during some bright flares and when the complete data sets are binned on long timescales. The main database consists of 878.4 hours of observation with the Whipple telescope, spread over 783 nights. The peak energy response of the telescope was 400 GeV with 20% uncertainty. This is the largest database of any TeV-emitting active galactic nucleus (AGN) and hence was used to explore the variability profile of Markarian 421. The time-averaged flux from Markarian 421 over this period was 0.446$\pm$0.008 Crab flux units. The flux exceeded 10 Crab flux units on three separate occasions. For the 2000-2001 season the average flux reached 1.86 Crab units, while in the 1996-1997 season the average flux was only 0.23 Crab units., Comment: 29 pages, 15 figures. Accepted for publication in Astroparticle Physics

Published
2014
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36. Mrk 421 active state in 2008: the MAGIC view, simultaneous multi-wavelength observations and SSC model constrained

Author

Jose Luis Contreras, Juan Cortina, J. Moldón, D. Hildebrand, L. Peruzzo, E. Lindfors, Diego F. Torres, R. Reinthal, M. Pasanen, J. Becerra González, M. Garczarczyk, L. Maraschi, Dominik Elsaesser, G. De Caneva, M. Doert, Michael Backes, Damir Lelas, Marc Ribó, Mario Meucci, Dorota Sobczyńska, Christian Fruck, A. Boller, L. Cossio, A. Carosi, C. Schultz, Kazuo Saito, Giacomo Bonnoli, P. G. Prada Moroni, V. Scalzotto, R. K. Bock, O. Tibolla, J. M. Paredes, Hiroko Miyamoto, T. Jogler, M. Shayduk, J. Krause, B. De Lotto, Jelena Aleksić, K. Nilsson, A. Herrero, M. Pilia, E. A. Alvarez, C. Delgado Mendez, D. Mazin, B. Huber, S. Partini, D. Höhne-Mönch, S. Klepser, E. De Cea del Pozo, Stefano Covino, J. Storz, P. Munar-Adrover, Daniel Ferenc, Denis Bastieri, P. Temnikov, E. Prandini, Andrei Berdyugin, Tihomir Surić, Patrick Vogler, David Paneque, H. Vankov, Nikola Godinovic, K. Berger, Aldo Treves, Abelardo Moralejo, E. Colombo, A. Saggion, P. Antoranz, Adrian Biland, H. Takami, Fabio Zandanel, Ilana M. Braun, R. Paoletti, Silvia Pardo, D. Tescaro, Daniela Dorner, M. Lopez, Karl Mannheim, J. Rico, V. Scapin, M. Asensio, Julian Sitarek, Ivica Puljak, J. Hose, Daniela Hadasch, L. O. Takalo, Miguel A. Pérez-Torres, Felix Spanier, Dario Hrupec, Reiko Orito, Wlodek Bednarek, T. Saito, Martin Makariev, Oscar Blanch, A. Lopez, Mosè Mariotti, M. Salvati, Roberta Zanin, D. Häfner, A. La Barbera, S. Spiro, E. Lorenz, A. Stamerra, D. Dominis Prester, A. Diago Ortega, Robert Wagner, T. Schweizer, R. J. García López, Elisa Bernardini, L. A. Antonelli, D. Garrido, A. Cañellas, Jose Miguel Miranda, Tomislav Terzić, Aaron Dominguez, Miriam Lucio Martinez, Francisco Prada, Felicitas Pauss, A. Sillanpää, A. De Angelis, Saverio Lombardi, Quirin Weitzel, Ignasi Reichardt, R. Mirzoyan, Victor Zabalza, Massimo Persic, Igor Oya, Fabrizio Tavecchio, Ll. Font, Daniel Nieto, J. Pochon, T. Krahenbuh, Thomas Bretz, Masahiro Teshima, Gianluca Giavitto, P. Colin, G. Maneva, Konstancja Satalecka, M. Uellenbeck, Michele Doro, N. Strah, E. Carmona, Wolfgang Rhode, Hanna Kellermann, Francesco Dazzi, E. Leonardo, N. Mankuzhiyil, M. V. Fonseca, S. N. Shore, B. Steinke, S. Rügamer, Juan Abel Barrio, and D. Borla Tridon

Subjects
Radiation mechanisms: non-thermal, ELECTRON ACCELERATION, BACKGROUND-RADIATION, Astrophysics, radiation mechanisms: non-thermal / BL Lacertae objects: individual: Mrk 421, 01 natural sciences, law.invention, Raigs gamma, law, Observacions astronòmiques, MAGIC (telescope), 010303 astronomy & astrophysics, media_common, High Energy Astrophysical Phenomena (astro-ph.HE), Physics, FAST TEV VARIABILITY, CORRELATED VARIABILITY, BL Lacertae objects: individual: Mrk 421, Astronomy and Astrophysics, Space and Planetary Science, SELF-COMPTON MODEL, symbols, Electrónica, Física nuclear, Electricidad, Astrophysics - High Energy Astrophysical Phenomena, Doppler effect, Astronomical observations, GALACTIC NUCLEI, media_common.quotation_subject, Astrophysics::High Energy Astrophysical Phenomena, FOS: Physical sciences, Context (language use), gamma ray astronomy, Cosmic rays, IMAGING CHERENKOV DETECTOR, Telescope, symbols.namesake, 0103 physical sciences, Blazar, Cherenkov radiation, BL-LACERTAE OBJECTS, GAMMA-RAY EMISSION, X-RAY, LAC OBJECTS, 010308 nuclear & particles physics, Gamma rays, Galaxies, Galàxies, Crab Nebula, 13. Climate action, Sky, ddc:520
Abstract

Context: The blazar Markarian 421 is one of the brightest TeV gamma-ray sources of the northern sky. From December 2007 until June 2008 it was intensively observed in the very high energy (VHE, E > 100 GeV) band by the single-dish Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC-I). Aims: We aimed to measure the physical parameters of the emitting region of the blazar jet during active states. Methods: We performed a dense monitoring of the source in VHE with MAGIC-I, and also collected complementary data in soft X-rays and optical-UV bands; then, we modeled the spectral energy distributions (SED) derived from simultaneous multi-wavelength data within the synchrotron self--compton (SSC) framework. Results: The source showed intense and prolonged gamma-ray activity during the whole period, with integral fluxes (E > 200 GeV) seldom below the level of the Crab Nebula, and up to 3.6 times this value. Eight datasets of simultaneous optical-UV (KVA, Swift/UVOT), soft X-ray (Swift/XRT) and MAGIC-I VHE data were obtained during different outburst phases. The data constrain the physical parameters of the jet, once the spectral energy distributions obtained are interpreted within the framework of a single-zone SSC leptonic model. Conclusions: The main outcome of the study is that within the homogeneous model high Doppler factors (40, 12 pages, 7 figures. Accepted for publication in A&A 7 January 2012

Published
2012
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37. TeV and Multi-wavelength Observations of Mrk 421 in 2006-2008

Author

Carolin Villforth, Matthias Beilicke, P. Kaaret, A. Pichel, M. Ouellette, S. Vincent, A. Pease, R. G. Wagner, Gernot Maier, J. E. Ward, C. M. Hui, Wystan Benbow, A. Sillanpää, Michael T. Carini, S. P. Wakely, Daniel Gall, M. McCutcheon, K. Ragan, A. Konopelko, Andrei Berdyugin, S. P. Swordy, D. S. Hanna, T. C. Arlen, A. Imran, S. McArthur, V. A. Acciari, R. Dickherber, M. Kertzman, N. Karlsson, D. Steele, Karen Byrum, T. B. Humensky, P. T. Reynolds, S. Thibadeau, E. Nieppola, E. Lindfors, G. H. Sembroski, M. Schroedter, J. S. Perkins, K. Nilsson, D. Boltuch, Hugh D. Aller, G. Demet Senturk, A. C. Rovero, R. Guenette, Geza Gyuk, P. Moriarty, Nicola Galante, Stephanie Wissel, Wei Cui, Amy Furniss, M. J. Lang, M. F. Aller, Alfred Garson, A. Cesarini, David A. Williams, T. Aune, L. O. Takalo, Jamie Holder, R. A. Ong, Thomas Weisgarber, G. H. Gillanders, S. Godambe, David Kieda, L. C. Reyes, John L. Quinn, H. J. Rose, G. Finnegan, C. Duke, Robert J. Walters, E. Roache, Matthew J.A. Wood, S. M. Bradbury, V. V. Vassiliev, Abraham D. Falcone, A. C. Sadun, Lucy Fortson, E. Aliu, Henric Krawczynski, A. J.R. Weinstein, A. Varlotta, L. Ciupik, J. Sainio, A. Cannon, D. Pandel, V. Bugaev, T. Montaruli, Talvikki Hovatta, M. Baker, Anne Lähteenmäki, Kyle Cook, B. Zitzer, J. Grube, S. Smith, J. P. Finley, T. C. Weekes, J. H. Buckley, F. Krennrich, M. Pasanen, M. Theiling, A. N. Otte, Merja Tornikoski, Jeremy D. Maune, D. Barnaby, M. Pohl, and K. Lee

Subjects
individual (Mrk 421) [BL Lacertae objects], galaxies [Gamma rays], Ciencias Físicas, Astrophysics::High Energy Astrophysical Phenomena, bl-lacertae objects, FOS: Physical sciences, Multi wavelength, Astrophysics::Cosmology and Extragalactic Astrophysics, Astrophysics, ray timing explorer, energy gamma-rays, 01 natural sciences, Spectral line, law.invention, Telescope, purl.org/becyt/ford/1 [https], Observatory, law, blazar markarian-421, 0103 physical sciences, gamma rays: galaxies, Blazar, 010303 astronomy & astrophysics, x-rays: galaxies, Cherenkov radiation, High Energy Astrophysical Phenomena (astro-ph.HE), Physics, particle-acceleration, 010308 nuclear & particles physics, Spectral density, Astronomy and Astrophysics, purl.org/becyt/ford/1.3 [https], galaxies: jets, Redshift, Astronomía, large-area telescope, x-ray, 13. Climate action, Space and Planetary Science, correlated variability, nuclei [Galaxies], active galactic nuclei, jets [Galaxies], bl lacertae objects: individual (mrk 421), galaxies: nuclei, Astrophysics - High Energy Astrophysical Phenomena, CIENCIAS NATURALES Y EXACTAS, log-parabolic spectra
Abstract

We report on TeV gamma-ray observations of the blazar Mrk 421 (redshift of 0.031) with the VERITAS observatory and the Whipple 10m Cherenkov telescope. The excellent sensitivity of VERITAS allowed us to sample the TeV gamma-ray fluxes and energy spectra with unprecedented accuracy where Mrk 421 was detected in each of the pointings. A total of 47.3 hrs of VERITAS and 96 hrs of Whipple 10m data were acquired between January 2006 and June 2008. We present the results of a study of the TeV gamma-ray energy spectra as a function of time, and for different flux levels. On May 2nd and 3rd, 2008, bright TeV gamma-ray flares were detected with fluxes reaching the level of 10 Crab. The TeV gamma-ray data were complemented with radio, optical, and X-ray observations, with flux variability found in all bands except for the radio waveband. The combination of the RXTE and Swift X-ray data reveal spectral hardening with increasing flux levels, often correlated with an increase of the source activity in TeV gamma-rays. Contemporaneous spectral energy distributions were generated for 18 nights, each of which are reasonably described by a one-zone SSC model., Comment: 30 pages, 12 figures, accepted by ApJ

Published
2011
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38. Multiwavelength Observations of the very high energy Blazar 1ES2344+514

Author

L. Ciupik, E. Aliu, R. Dickherber, P. Kaaret, R. Guenette, B. Zitzer, J. E. Ward, Gernot Maier, S. P. Swordy, A. Varlotta, S. P. Wakely, D. Boltuch, A. N. Otte, D. S. Hanna, T. C. Arlen, A. Imran, Daniel Gall, Amanda Weinstein, A. Cannon, Thomas Brian Humensky, P. Cogan, M. Schroedter, Lucy Fortson, R. A. Ong, F. Krennrich, Matthew Wood, D. Pandel, Vladimir Vassiliev, Pierre Colin, S. LeBohec, J. S. Perkins, K. Ragan, M. Theiling, Henric Krawczynski, A. Konopelko, J. P. Finley, A. W. Smith, Amy Furniss, Reshmi Mukherjee, Matthias Beilicke, J. A. Toner, T. C. Weekes, Martin Pohl, G. H. Sembroski, A. D. Falcone, P. Moriarty, John Kildea, Stephanie Wissel, David Kieda, T. Weisgarber, J. Grube, Jamie Holder, P. T. Reynolds, Rick Wagner, S. Vincent, David A. Williams, C. M. Hui, Wystan Benbow, Geza Gyuk, V. Bugaev, V. A. Acciari, P. Fortin, H. J. Rose, S. J. Fegan, D. Steele, M. J. Lang, A. Pichel, T. Aune, N. Karlsson, John L. Quinn, D. Horan, M. Kertzman, and G. H. Gillanders

Subjects
Photon, Ciencias Físicas, bl-lacertae objects, Astrophysics, 01 natural sciences, Spectral line, law.invention, Telescope, purl.org/becyt/ford/1 [https], law, veritas, 0103 physical sciences, galaxies, strong flares, individual, Blazar, 010303 astronomy & astrophysics, Physics, markarian 501, 1ES 2344+514, 010308 nuclear & particles physics, spectral variability, Astronomy and Astrophysics, purl.org/becyt/ford/1.3 [https], gamma-ray emission, Galaxy, Massless particle, bl lacertae objects: individual (ies 2344+514), radiation, Astronomía, einstein slew survey, Crab Nebula, 13. Climate action, Space and Planetary Science, correlated variability, cherenkov telescopes, BL Lacertae objects, CIENCIAS NATURALES Y EXACTAS, Flare
Abstract

Multiwavelength observations of the high-frequency-peaked blazar 1ES 2344+514 were performed from October 2007 to January 2008. The campaign represents the first contemporanous data on the object at very high energy (VHE, E > 100 GeV) gamma-ray, X-ray, and UV energies. Observations with VERITAS in VHE gamma-rays yield a strong detection of 20 sigmas with 633 excess events in a total exposure of 18.1 hours live-time. A strong VHE gamma-ray flare on December 7, 2007 is measured at F(>300 GeV) = (6.76 ± 0.62) × 10^-11 cm^-2 s^-1 , corresponding to 48% of the Crab Nebula flux. Excluding this flaring episode, nightly variability at lower fluxes is observed with a time-averaged mean of F(>300 GeV) = (1.06 ± 0.09) × 10^-11 cm^-2 s^-1 (7.6% of the Crab Nebula flux). The differential photon spectrum between 390 GeV and 8.3 TeV for the time-averaged observations excluding December 7, 2007 is well described by a power law with a photon index of 2.78 ± 0.09 (stat) ± 0.15 (syst). A slightly harder photon index of 2.43 ± 0.22stat ± 0.15syst is measured on the flaring night of December 7, 2007. Over the full period of VERITAS observations contemporaneous X-ray and UV data were taken with Swift and RXTE. Variability by a factor of ~7 is evident in the 2-10 keV flux between nightly observations. On December 8, 2007 the highest ever observed X-ray flux from 1ES 2344+514 was measured by Swift XRT at a flux of F(2-10 keV) = (6.58 ± 0.37) × 10^-11 erg cm^-2 s^-1. Evidence for a correlation between the X-ray flux and VHE gamma-ray flux on nightly time-scales is indicated with a Pearson correlation coefficient of r = 0.60 ± 0.11. Contemporaneous spectral energy distributions (SEDs) of 1ES 2344+514 are presented for two distinct flux states. A one-zone synchrotron self-Compton (SSC) model describes both SEDs using parameters consistent with previous SSC modeling of 1ES 2344+514 from non-contemporaneous observations. Fil: Acciari, V. A.. Harvard-Smithsonian Center for Astrophysics; Estados Unidos Fil: Aliu, E.. University of Delaware; Estados Unidos Fil: Arlen, T.. University of California; Estados Unidos Fil: Aune, T.. University of California; Estados Unidos Fil: Beilicke, M.. University of Washington; Estados Unidos Fil: Benbow, W.. Harvard-Smithsonian Center for Astrophysics; Estados Unidos Fil: Boltuch, D.. University of Delaware; Estados Unidos Fil: Bugaev, V.. University of Washington; Estados Unidos Fil: Cannon, A.. University College Dublin; Irlanda Fil: Ciupik, L.. Adler Planetarium and Astronomy Museum; Estados Unidos Fil: Cogan, P.. McGill University; Canadá Fil: Colin, P.. University of Utah; Estados Unidos Fil: Dickherber, R.. University of Washington; Estados Unidos Fil: Falcone, A.. University of Pennsylvania; Estados Unidos Fil: Fegan, S. J.. University of California; Estados Unidos Fil: Finley, J. P.. Purdue University; Estados Unidos Fil: Fortin, P.. Columbia University; Estados Unidos Fil: Fortson, L. F.. Adler Planetarium and Astronomy Museum; Estados Unidos Fil: Furniss, A.. University of California; Estados Unidos Fil: Gall, D.. Purdue University; Estados Unidos Fil: Gillanders, G. H.. National University of Ireland; Irlanda Fil: Grube, J.. University College Dublin; Irlanda Fil: Guenette, R.. McGill University; Canadá Fil: Gyup, G.. Adler Planetarium and Astronomy Museum; Estados Unidos Fil: Hanna, D.. McGill University; Canadá Fil: Holder, J.. University of Delaware; Estados Unidos Fil: Horan, D.. Ecole Polytechnique; Francia Fil: Hui, C. M.. University of Utah; Estados Unidos Fil: Humensky, T. B.. University Of Chicago; Estados Unidos Fil: Pichel, Ana Carolina. Consejo Nacional de Investigaciónes Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Astronomía y Física del Espacio. - Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Astronomía y Física del Espacio; Argentina

Published
2011
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39. MAGIC TeV gamma-ray observations of Markarian 421 during multiwavelength campaigns in 2006

Author

Pierre Colin, R. J. García López, Agnieszka Sierpowska-Bartosik, Jose Luis Contreras, A. De Angelis, Juan Cortina, Massimo Persic, C. Delgado Mendez, E. Prandini, S. Balestra, Marc Ribó, P. Antoranz, Dorota Sobczyńska, S. Rügamer, M. T. Costado, Thomas Bretz, Daniel Nieto, S. N. Shore, Juan Abel Barrio, Abelardo Moralejo, D. Borla Tridon, L. O. Takalo, K. Berger, Jelena Ninkovic, Stefano Covino, Valentí Bosch-Ramon, I. Puljak, C. C. Hsu, Kari Nilsson, V. Scapin, Mosè Mariotti, Stefan Klepser, Miguel A. Pérez-Torres, D. Hadasch, Laura Maraschi, Nikola Godinovic, M. Doert, Michael Backes, M. De Maria, D. Tescaro, D. Elsaesser, Denis Bastieri, Felix Spanier, M. Camara, V. Scalzotto, Tihomir Surić, Tomislav Terzić, Wolfgang Rhode, Francesco Dazzi, Daniela Dorner, Miguel A. Sánchez-Conde, M. I. Martínez, Riccardo Paoletti, Saverio Lombardi, Daniel Ferenc, J. C. Struebig, D. Bose, E. Leonardo, H. Vankov, Ilana M. Braun, D. Dominis Prester, Pratik Majumdar, Roberta Zanin, M. V. Fonseca, M. Rissi, Elina Lindfors, Daniel Mazin, J. M. Paredes, Antonio Stamerra, D. Britzger, S. Paiano, Petar Temnikov, Giacomo Bonnoli, Diego F. Torres, M. Pasanen, R. G. Pegna, Karl Mannheim, T. Krähenbühl, Francisco Prada, Elisa Bernardini, A. Laille, R. Mirzoyan, Andrei Berdyugin, B. Steinke, M. López, E. Carmona, Markus Garczarczyk, S. Partini, Francesco Longo, D. Höhne-Mönch, N. Puchades, J. Becerra González, B. De Lotto, R. de los Reyes, D. Pascoli, Takashi Saito, F. De Sabata, T. Schweizer, Javier Moldon, E. De Cea del Pozo, Mario Meucci, C. Schultz, Pol Bordas, Aaron Dominguez, Felicitas Pauss, S. Spiro, E. Lorenz, Reiko Orito, Victor Zabalza, A. Herrero, L. A. Antonelli, M. Shayduk, Mariano Moles, J. Rico, Igor Oya, Fabrizio Tavecchio, Daniel Kranich, Nijil Mankuzhiyil, Ll. Font, R. K. Bock, T. Jogler, A. La Barbera, D. Hildebrand, L. Peruzzo, M. Salvati, A. Sillanpää, Dario Hrupec, Masahiro Teshima, Manel Errando, Fabio Zandanel, Konstancja Satalecka, Michele Doro, Robert Wagner, Hiroko Miyamoto, Jelena Aleksić, C. Baixeras, Julian Sitarek, J. Hose, A. Saggion, Adrian Biland, Markus Gaug, Ignasi Reichardt, G. Maneva, S. Commichau, Wlodek Bednarek, Alessandro Carosi, Jose Miguel Miranda, Universitat de Barcelona, J., Aleksic, H., Anderhub, L. A., Antonelli, P., Antoranz, M., Backe, C., Baixera, S., Balestra, J. A., Barrio, D., Bastieri, J. B., Gonzalez, J. K., Becker, W., Bednarek, A., Berdyugin, K., Berger, E., Bernardini, A., Biland, R. K., Bock, G., Bonnoli, P., Borda, D. B., Tridon, V., Bosch Ramon, D., Bose, I., Braun, T., Bretz, D., Britzger, M., Camara, E., Carmona, A., Carosi, P., Colin, S., Commichau, J. L., Contrera, J., Cortina, M. T., Costado, S., Covino, F., Dazzi, A. D., Angeli, E. D., del Pozo, R. D., Reye, B. D., Lotto, M. D., Maria, F. D., Sabata, C. D., Mendez, M., Doert, A., Dominguez, D. D., Prester, D., Dorner, M., Doro, D., Elsaesser, M., Errando, D., Ferenc, M. V., Fonseca, L., Font, R. J. G., Lopez, M., Garczarczyk, M., Gaug, N., Godinovic, D., Hadasch, A., Herrero, D., Hildebrand, D., Hohne Monch, J., Hose, D., Hrupec, C. C., Hsu, T., Jogler, S., Klepser, T., Krahenbuhl, D., Kranich, A. L., Barbera, A., Laille, E., Leonardo, E., Lindfor, S., Lombardi, Longo, Francesco, M., Lopez, E., Lorenz, P., Majumdar, G., Maneva, N., Mankuzhiyil, K., Mannheim, L., Maraschi, M., Mariotti, M., Martinez, D., Mazin, M., Meucci, J. M., Miranda, R., Mirzoyan, H., Miyamoto, J., Moldon, M., Mole, A., Moralejo, D., Nieto, K., Nilsson, J., Ninkovic, R., Orito, I., Oya, R., Paoletti, J. M., Parede, S., Partini, M., Pasanen, D., Pascoli, F., Pau, R. G., Pegna, M. A., Perez Torre, M., Persic, L., Peruzzo, F., Prada, E., Prandini, N., Puchade, I., Puljak, I., Reichardt, W., Rhode, M., Ribo, J., Rico, M., Rissi, S., Rugamer, A., Saggion, T. Y., Saito, M., Salvati, M., Sanchez Conde, K., Satalecka, V., Scalzotto, V., Scapin, T., Schweizer, M., Shayduk, S. N., Shore, A., Sierpowska Bartosik, A., Sillanpaa, J., Sitarek, D., Sobczynska, F., Spanier, S., Spiro, A., Stamerra, B., Steinke, N., Strah, J. C., Struebig, T., Suric, L., Takalo, F., Tavecchio, P., Temnikov, D., Tescaro, M., Teshima, D. F., Torre, H., Vankov, R. M., Wagner, V., Zabalza, F., Zandanel, and R., Zanin

Subjects
galaxies [Gamma rays], PARTICLE-ACCELERATION, Radiation mechanisms: non-thermal, Flux, Astrophysics, 01 natural sciences, Spectral line, law.invention, Raigs gamma, ACTIVE GALAXY MARKARIAN-421, law, Jets (Astrofísica), MAGIC (telescope), 010303 astronomy & astrophysics, Palma (Canary Islands), BL Lacertae objects: individual: Mkn 421, Physics, Spectral index, Palma (Canàries), Very High Energy Gamma-ray Astronomy, MAGIC telescope, Active Galactic Nuclei, Gamma ray, CORRELATED VARIABILITY, Gamma rays: galaxies, astro-ph.CO, Electrónica, Física nuclear, Electricidad, Telescopis espacials, Astrophysics - Cosmology and Nongalactic Astrophysics, Cosmology and Nongalactic Astrophysics (astro-ph.CO), gamma ray astronomy, IMAGING CHERENKOV DETECTOR, gamma rays: observations, BL Lacertae objects: individual (Mkn 421), FOS: Physical sciences, BLAZAR MARKARIAN-421, Space telescopes, Telescope, BL-LAC OBJECTS, 0103 physical sciences, SPECTRA, Nucli galàctic actiu, Blazar, individual: Mkn 421 [BL Lacertae objects], ATMOSPHERIC CHERENKOV TELESCOPE, HIGH-ENERGY PHOTONS, X-RAY, CRAB-NEBULA, Active galactic nuclei, non-thermal [Radiation mechanisms], 010308 nuclear & particles physics, Gamma rays, Astronomy and Astrophysics, Light curve, Galaxies, Galàxies, Astrophysical jets, 13. Climate action, Space and Planetary Science, ddc:520
Abstract

10 páginas, 7 figuras, 6 tablas.-- El Pdf del artículo es la versión pre-print: arXiv:1001.1291.-- MAGIC Collaboration: et al., [Context]: Wide-range spectral coverage of blazar-type active galactic nuclei is of paramount importance for understanding the particle acceleration mechanisms assumed to take place in their jets. The Major Atmospheric Gamma Imaging Cerenkov (MAGIC) telescope participated in three multiwavelength (MWL) campaigns, observing the blazar Markarian (Mkn) 421 during the nights of April 28 and 29, 2006, and June 14, 2006. [Aims]: We analyzed the corresponding MAGIC very-high energy observations during 9 nights from April 22 to 30, 2006 and on June 14, 2006. We inferred light curves with sub-day resolution and night-by-night energy spectra. [Methods]: MAGIC detects γ-rays by observing extended air showers in the atmosphere. The obtained air-shower images were analyzed using the standard MAGIC analysis chain. [Results]: A strong γ-ray signal was detected from Mkn 421 on all observation nights. The flux (E > 250 GeV) varied on night-by-night basis between (0.92±0.11) × 10-10 cm-2 s-1 (0.57 Crab units) and (3.21±0.15) × 10-10 cm-2 s-1 (2.0 Crab units) in April 2006. There is a clear indication for intra-night variability with a doubling time of 36± min on the night of April 29, 2006, establishing once more rapid flux variability for this object. For all individual nights γ-ray spectra could be inferred, with power-law indices ranging from 1.66 to 2.47. We did not find statistically significant correlations between the spectral index and the flux state for individual nights. During the June 2006 campaign, a flux substantially lower than the one measured by the Whipple 10-m telescope four days later was found. Using a log-parabolic power law fit we deduced for some data sets the location of the spectral peak in the very-high energy regime. Our results confirm the indications of rising peak energy with increasing flux, as expected in leptonic acceleration models., The support of the German BMBF and MPG, the Italian INFN and Spanish MICINN is gratefully acknowledged. This work was also supported by ETH Research Grant TH 34/043, by the Polish MNiSzW Grant N N203 390834, and by the YIP of the Helmholtz Gemeinschaft.

Published
2010
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40. Multiwavelength observations of markarian 421 in 2005-2006

Author

D. Horan, V. A. Acciari, S. M. Bradbury, J. H. Buckley, V. Bugaev, K. L. Byrum, A. Cannon, O. Celik, A. Cesarini, Y. C. K. Chow, L. Ciupik, P. Cogan, A. D. Falcone, S. J. Fegan, J. P. Finley, P. Fortin, L. F. Fortson, D. Gall, G. H. Gillanders, J. Grube, G. Gyuk, D. Hanna, E. Hays, M. Kertzman, J. Kildea, A. Konopelko, H. Krawczynski, F. Krennrich, M. J. Lang, K. Lee, P. Moriarty, T. Nagai, J. Niemiec, R. A. Ong, J. S. Perkins, M. Pohl, J. Quinn, P. T. Reynolds, H. J. Rose, G. H. Sembroski, A. W. Smith, D. Steele, S. P. Swordy, J. A. Toner, V. V. Vassiliev, S. P. Wakely, T. C. Weekes, R. J. White, D. A. Williams, M. D. Wood, B. Zitzer, H. D. Aller, M. F. Aller, M. Baker, D. Barnaby, M. T. Carini, P. Charlot, J. P. Dumm, N. E. Fields, T. Hovatta, B. Jordan, Y. A. Kovalev, Y. Y. Kovalev, H. A. Krimm, O. M. Kurtanidze, A. Lähteenmäki, J. F. Le Campion, J. Maune, T. Montaruli, A. C. Sadun, S. Smith, M. Tornikoski, M. Turunen, and R. Walters

Subjects
spectral slope variability, bl-lacertae objects, FOS: Physical sciences, bl lacertae objects: individual (markarian 421), proton blazar, tev blazar markarian-421, Astrophysics, timing explorer, Radio spectrum, law.invention, Telescope, Observatory, law, Blazar, Physics, High Energy Astrophysical Phenomena (astro-ph.HE), Northern Hemisphere, Astronomy and Astrophysics, gamma-ray emission, lac objects, Wavelength, x-ray, Space and Planetary Science, correlated variability, active galactic nuclei, Spectral energy distribution, Astrophysics - High Energy Astrophysical Phenomena, gamma rays: observations, x-rays: individual (markarian 421)
Abstract

Since September 2005, the Whipple 10m Gamma-ray Telescope has been operated primarily as a blazar monitor. The five Northern Hemisphere blazars that have already been detected at the Whipple Observatory, Markarian 421, H1426+428, Markarian 501, 1ES 1959+650 and 1ES 2344+514, are monitored routinely each night that they are visible. We report on the Markarian 421 observations taken from November 2005 to June 2006 in the gamma-ray, X-ray, optical and radio bands. During this time, Markarian 421 was found to be variable at all wavelengths probed. Both the variability and the correlations among different energy regimes are studied in detail here. A tentative correlation, with large spread, was measured between the X-ray and gamma-ray bands, while no clear correlation was evident among the other energy bands. In addition to this, the well-sampled spectral energy distribution of Markarian 421 (1101+384) is presented for three different activity levels. The observations of the other blazar targets will be reported separately., Comment: 58 pages, 22 Figures. Accepted for publication in ApJ

Published
2009

41. SIMULTANEOUS MULTIWAVELENGTH OBSERVATIONS OF MARKARIAN 421 DURING OUTBURST

Author

Acciari, Va, Aliu, E, Aune, T, Beilicke, M, Benbow, W, Bottcher, M, Bradbury, Sm, Buckley, Jh, Bugaev, V, Butt, Y, Cannon, A, Celik, O, Cesarini, A, Chow, Yc, Ciupik, L, Cogan, P, Colin, P, Cui, W, Dickherber, R, Duke, C, Falcone, Ad, Fegan, Sj, Finley, Jp, Finnegan, G, Fortin, P, Fortson, L, Furniss, A, Gall, D, Gillanders, Gh, Grube, J, Guenette, R, Gyuk, G, Hanna, D, Holder, J, Horan, D, Hui, Cm, Humensky, Tb, Kaaret, P, Karlsson, N, Kertzman, M, Kieda, D, Kildea, J, Konopelko, A, Krawczynski, H, Krennrich, F, Lang, Mj, Lebohec, S, Maier, G, Mccann, A, Millis, J, Moriarty, P, Ong, Ra, Otte, An, Pandel, D, Perkins, Js, Pichel, A, Pohl, M, Quinn, J, Ragan, K, Reyes, Lc, Reynolds, Pt, Roache, E, Rose, Hj, Schroedter, M, Sembroski, Gh, Smith, Aw, Steele, D, Swordy, Sp, Theiling, M, Toner, Ja, Varlotta, A, Vincent, S, Wakely, Sp, Ward, Je, Weekes, Tc, Weinstein, A, Weisgarber, T, Williams, Da, Wissel, S, Zitzer, B, Perez, Id, Ibarra, A, Anderhub, Prh, Antonelli, La, Antoranz, P, Backes, M, Baixeras, C, Balestra, S, Barrio, Ja, Bastieri, D, Gonzalez, Jb, Becker, Jk, Bednarek, W, Berger, K, Bernardini, E, Biland, A, Bock, Rk, Bonnoli, G, Bordas, P, Tridon, Db, Bosch Ramon, V, Bose, D, Braun, I, Bretz, T, Britvitch, I, Camara, M, Carmona, E, Carosi, A, Commichau, S, Contreras, Jl, Cortina, J, Costado, Mt, Covino, S, Curtef, V, Dazzi, F, DE ANGELIS, Alessandro, Del Pozo ED, Mendez, Cd, De Los Reyes, R, DE LOTTO, Barbara, De Maria, M, De Sabata, F, Dominguez, A, Dorner, D, Doro, M, Elsaesser, D, Errando, M, Ferenc, D, Fernandez, E, Firpo, R, Fonseca, Mv, Font, L, Galante, N, Lopez, Rj, Garczarczyk, M, Gaug, M, Goebel, F, Hadasch, D, Hayashida, M, Herrero, A, Hildebrand, D, Hohne Monch, D, Hose, J, Hsu, Cc, Jogler, T, Kranich, D, La Barbera, A, Laille, A, Leonardo, E, Lindfors, E, Lombardi, S, Longo, F, Lopez, M, Lorenz, E, Majumdar, P, Maneva, G, Mankuzhiyil, N, Mannheim, K, Maraschi, L, Mariotti, M, Martinez, M, Mazin, D, Meucci, M, Miranda, Jm, Mirzoyan, R, Miyamoto, H, Moldon, J, Moles, M, Moralejo, A, Nieto, D, Nilsson, K, Ninkovic, J, Orito, R, Oya, I, Paoletti, R, Paredes, Jm, Pasanen, M, Pascoli, D, Pauss, F, Pegna, Rg, Perez Torres MA, Persic, M, Peruzzo, L, Prada, F, Prandini, E, Puchades, N, Reichardt, I, Rhode, W, Ribo, M, Rico, J, Rissi, M, Robert, A, Rugamer, S, Saggion, A, Saito, Ty, Salvati, M, Sanchez Conde, M, Satalecka, K, Scalzotto, V, Scapin, V, Schweizer, T, Shayduk, M, Shore, Sn, Sidro, N, Sierpowska Bartosik, A, Silanpaa, A, Sitarek, J, Sobczynska, D, Spanier, F, Spiro, S, Stamerra, A, Stark, Ls, Takalo, L, Tavecchio, F, Temnikov, P, Tescaro, D, Teshima, M, Tluczykont, M, Torres, Df, Turini, N, Vankov, H, Wagner, Rm, Zabalza, V, Zandanel, F, Zanin, R, Zapatero, J., V. A., Acciari, E., Aliu, T., Aune, M., Beilicke, W., Benbow, M., Bottcher, S. M., Bradbury, J. H., Buckley, V., Bugaev, Y., Butt, A., Cannon, O., Celik, A., Cesarini, Y. C., Chow, L., Ciupik, P., Cogan, P., Colin, W., Cui, R., Dickherber, C., Duke, A. D., Falcone, S. J., Fegan, J. P., Finley, G., Finnegan, P., Fortin, L., Fortson, A., Furni, D., Gall, G. H., Gillander, J., Grube, R., Guenette, G., Gyuk, D., Hanna, J., Holder, D., Horan, C. M., Hui, T. B., Humensky, P., Kaaret, N., Karlsson, M., Kertzman, D., Kieda, J., Kildea, A., Konopelko, H., Krawczynski, F., Krennrich, M. J., Lang, S., Lebohec, G., Maier, A., Mccann, J., Milli, P., Moriarty, R. A., Ong, A. N., Otte, D., Pandel, J. S., Perkin, A., Pichel, M., Pohl, J., Quinn, K., Ragan, L. C., Reye, P. T., Reynold, E., Roache, H. J., Rose, M., Schroedter, G. H., Sembroski, A. W., Smith, D., Steele, S. P., Swordy, M., Theiling, J. A., Toner, A., Varlotta, S., Vincent, S. P., Wakely, J. E., Ward, T. C., Weeke, A., Weinstein, T., Weisgarber, D. A., William, S., Wissel, B., Zitzer, I. D., Perez, A., Ibarra, P. R. H., Anderhub, L. A., Antonelli, P., Antoranz, M., Backe, C., Baixera, S., Balestra, J. A., Barrio, D., Bastieri, J. B., Gonzalez, J. K., Becker, W., Bednarek, K., Berger, E., Bernardini, A., Biland, R. K., Bock, G., Bonnoli, P., Borda, D. B., Tridon, V., Bosch Ramon, D., Bose, I., Braun, T., Bretz, I., Britvitch, M., Camara, E., Carmona, A., Carosi, S., Commichau, J. L., Contrera, J., Cortina, M. T., Costado, S., Covino, V., Curtef, F., Dazzi, A. D., Angeli, E. D., Del Pozo, C. D., Mendez, R. D., Los Reye, B. D., Lotto, M. D., Maria, F. D., Sabata, A., Dominguez, D., Dorner, M., Doro, D., Elsaesser, M., Errando, D., Ferenc, E., Fernandez, R., Firpo, M. V., Fonseca, L., Font, N., Galante, R. J., Lopez, M., Garczarczyk, M., Gaug, F., Goebel, D., Hadasch, M., Hayashida, A., Herrero, D., Hildebrand, D., Hohne Monch, J., Hose, C. C., Hsu, T., Jogler, D., Kranich, A. L., Barbera, A., Laille, E., Leonardo, E., Lindfor, S., Lombardi, Longo, Francesco, M., Lopez, E., Lorenz, P., Majumdar, G., Maneva, N., Mankuzhiyil, K., Mannheim, L., Maraschi, M., Mariotti, M., Martinez, D., Mazin, M., Meucci, J. M., Miranda, R., Mirzoyan, H., Miyamoto, J., Moldon, M., Mole, A., Moralejo, D., Nieto, K., Nilsson, J., Ninkovic, R., Orito, I., Oya, R., Paoletti, J. M., Parede, M., Pasanen, D., Pascoli, F., Pau, R. G., Pegna, M. A., Perez Torre, M., Persic, L., Peruzzo, F., Prada, E., Prandini, N., Puchade, I., Reichardt, W., Rhode, M., Ribo, J., Rico, M., Rissi, A., Robert, S., Rugamer, A., Saggion, T. Y., Saito, M., Salvati, M., Sanchez Conde, K., Satalecka, V., Scalzotto, V., Scapin, T., Schweizer, M., Shayduk, S. N., Shore, N., Sidro, A., Sierpowska Bartosik, A., Silanpaa, J., Sitarek, D., Sobczynska, F., Spanier, S., Spiro, A., Stamerra, L. S., Stark, L., Takalo, F., Tavecchio, P., Temnikov, D., Tescaro, M., Teshima, M., Tluczykont, D. F., Torre, N., Turini, H., Vankov, R. M., Wagner, V., Zabalza, F., Zandanel, R., Zanin, J., Zapatero, and Universitat de Barcelona

Subjects
bl lacertae objects: individual (markarian 421), 01 natural sciences, law.invention, ENERGY, Raigs gamma, ACTIVE GALAXY MARKARIAN-421, law, Observacions astronòmiques, RAY SPECTRAL VARIABILITY, 010303 astronomy & astrophysics, High Energy Astrophysical Phenomena (astro-ph.HE), Physics, Very High Energy Gamma-ray Astronomy, MAGIC telescope, Active Galactic Nuclei, Astrophysics::Instrumentation and Methods for Astrophysics, CORRELATED VARIABILITY, TEV PHOTONS, Wavelength, Electrónica, Física nuclear, Raigs X, Electricidad, Astrophysics - High Energy Astrophysical Phenomena, Astronomical observations, GALACTIC NUCLEI, Astrophysics::High Energy Astrophysical Phenomena, galaxies: active, FOS: Physical sciences, Telescope, 0103 physical sciences, X-rays, Blazar, x-rays: galaxies, XMM-NEWTON, 010308 nuclear & particles physics, Gamma rays, GAMMA-RAYS, Astronomy, Spectral density, Astronomy and Astrophysics, radiation mechanisms: non-thermal, Galaxies, Galàxies, MAGIC TELESCOPE, RADIATION, 13. Climate action, Space and Planetary Science, Satellite, gamma rays: observations
Abstract

We report on the results of two coordinated multiwavelength campaigns that focused on the blazar Markarian 421 during its 2006 and 2008 outbursts. These campaigns obtained UV and X-ray data using the XMM-Newton satellite, while the gamma-ray data were obtained utilizing three imaging atmospheric Cerenkov telescopes, the Whipple 10m telescope and VERITAS, both based in Arizona, as well as the MAGIC telescope, based on La Palma in the Canary Islands. The coordinated effort between the gamma-ray groups allowed for truly simultaneous data in UV/X-ray/gamma-ray wavelengths during a significant portion of the XMM-Newton observations. This simultaneous coverage allowed for a reliable search for correlations between UV, X-ray and gamma-ray variability over the course of the observations. Investigations of spectral hysteresis and modeling of the spectral energy distributions are also presented., Comment: 27 pages, 5 figures, Accepted for publication in The Astrophysical Journal

Published
2009
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42. Cortical activity is more stable when sensory stimuli are consciously perceived.

Author

Schurger A, Sarigiannidis I, Naccache L, Sitt JD, and Dehaene S

Subjects
Adult, Consciousness Disorders physiopathology, Evoked Potentials physiology, Female, Humans, Male, Perception, Physical Stimulation, Reproducibility of Results, Task Performance and Analysis, Time Factors, Young Adult, Cerebral Cortex physiopathology, Consciousness physiology, Sensation physiology
Abstract

According to recent evidence, stimulus-tuned neurons in the cerebral cortex exhibit reduced variability in firing rate across trials, after the onset of a stimulus. However, in order for a reduction in variability to be directly relevant to perception and behavior, it must be realized within trial--the pattern of activity must be relatively stable. Stability is characteristic of decision states in recurrent attractor networks, and its possible relevance to conscious perception has been suggested by theorists. However, it is difficult to measure on the within-trial time scales and broadly distributed spatial scales relevant to perception. We recorded simultaneous magneto- and electroencephalography (MEG and EEG) data while subjects observed threshold-level visual stimuli. Pattern-similarity analyses applied to the data from MEG gradiometers uncovered a pronounced decrease in variability across trials after stimulus onset, consistent with previous single-unit data. This was followed by a significant divergence in variability depending upon subjective report (seen/unseen), with seen trials exhibiting less variability. Applying the same analysis across time, within trial, we found that the latter effect coincided in time with a difference in the stability of the pattern of activity. Stability alone could be used to classify data from individual trials as "seen" or "unseen." The same metric applied to EEG data from patients with disorders of consciousness exposed to auditory stimuli diverged parametrically according to clinically diagnosed level of consciousness. Differences in signal strength could not account for these results. Conscious perception may involve the transient stabilization of distributed cortical networks, corresponding to a global brain-scale decision.

Published
2015
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