Search

We present the results of the gamma-ray flux distribution analysis on 145 gamma-ray bright blazars observed by Fermi-LAT. For the gamma-ray flux distribution, we applied a log-normal distribution to discuss the nature of the high-energy emission processes of blazars and a power-law distribution convolved with a Poisson distribution to investigate the implications of gamma-ray bright blazars for neutrino emission. Both distributions can represent the observed flux distributions as well. The leptonic models, which give the physical relationship between neutrinos and gamma rays, indicate that the flaring contribution to the neutrino emission can be dominant for the power-law index less than ∼2.5. From the power-law distribution analysis, we found that the power-law index < 2.5 accounts for the 82 % blazars. This result suggests that the flaring contribution of blazars is dominant for high-energy neutrino emission. [ABSTRACT FROM AUTHOR]

The recent status of the European Space Agency's (ESA) THESEUS (Transient High‐Energy Sources and Early Universe Surveyor) project and the recent and expected Czech participation in this project are briefly presented and discussed. THESEUS is a mission concept proposed in response to the ESA's call for medium‐size mission (M5) within the Cosmic Vision Programme and selected by ESA on May 7, 2018, to enter an assessment phase A/0 study. The expected Czech participation is on the Soft X‐ray Imager (SXI) and a set of four lobster‐eye (LE) telescopes, covering a total field of view (FOV) of 1 sr with source location accuracy better than 1–2 arcmin and energy coverage 0.3–6 keV. We also show the prospects of detecting and observing cataclysmic variables (CVs) with the hard X‐ray imaging spectrometer (XGIS) and SXI. Since the X‐ray spectrum largely determines what emission components and mechanisms can be observed, a combination of these instruments will be very useful. Also if we concentrate on the long‐term activity of CVs, it will even be meaningful to use the observations of CVs located in the wide FOVs of XGIS and SXI when searching for and observing other types of events such as gamma‐ray bursts. [ABSTRACT FROM AUTHOR]

The origin of the diffuse flux of very high-energy cosmic neutrinos above TeV energies observed by the IceCube experiment at the South Pole is still largely unknown. Simultaneous multi-frequency observations of the uncertainty location regions of these cosmic neutrinos are needed to identify the possible electromagnetic counterpart. Since 2016, the IceCube collaboration alerts almost in real time the astronomical community whenever a clear signature of a neutrino-induced event is recorded. The AGILE γ -ray satellite is fully involved in this multi-messenger hunt for cosmic neutrino sources. Using data obtained by the γ -ray imager onboard of the satellite, we searched for transient gamma-ray sources above 100 MeV that are temporally and spatially coincident with recent high-energy neutrino IceCube events. We find three AGILE candidate sources that can be considered as possible counterparts to neutrino events. The chance probability of this association is shown to be very low. One of the sources detected by AGILE in γ rays is the blazar TXS 0506+056, recently suggested as the first most likely extra-galactic emitter of TeV neutrinos. For the other two gamma-ray sources there are no obvious known counterparts, and both galactic and extragalactic origin should be considered. [ABSTRACT FROM AUTHOR]

We present an overview of the main AGILE data center activities and architecture. AGILE is a space mission of the Italian Space Agency (ASI) in joint collaboration with INAF, INFN, CIFS, and with the participation of several Italian space industry companies. The AGILE satellite was launched on April 23, 2007, and is devoted to the observation of the gamma-ray Universe in the 30 MeV–50 GeV energy range, with simultaneous X-ray imaging capability in the 18–60 keV band. The AGILE Data Center, part of the ASI multi-mission Space Science Data Center (SSDC, previously known as ASDC) is in charge of all the scientific operations: data management, archiving, distribution of AGILE data and scientific software, and user support. Thanks to its sky monitoring capability and fast ground segment alert system, AGILE is substantially improving our knowledge of the gamma-ray sky, and provides a crucial contribution to multimessenger follow-up of gravitational waves and neutrinos. [ABSTRACT FROM AUTHOR]

AGILE is a space mission of the Italian Space Agency dedicated to X-ray and gamma-ray astrophysics operating since 2007. Since the discovery of gravitational wave (GW) events by the LIGO-Virgo Collaboration (LVC) announced in February 2016, AGILE is playing a very important role in the search for possible X-ray and gamma-ray counterparts. The large fields of view of the gamma-ray and hard X-ray imagers (2.5 sr and 1 sr, respectively) and the current spinning mode allow AGILE to cover at any moment a very large fraction of the sky. We present here an overview of AGILE observations of GW events during the LVC observing periods O1 and O2 which span the time interval September 2015–August 2017. In particular, we focus on the main characteristics of AGILE observations of the events GW150914, GW170104, and GW170817. The latter event is of great relevance being associated with a "kilonova" counterpart: we establish important physical constraints on the possible γ -ray emission from a magnetar-like remnant in the first ∼ 1000 s following the GW event time. We also present further developments and preliminary results obtained in preparation of the O3 observing run (start in April 2019). AGILE is fully operative in the search for GW event counterparts, and continues to observe the sky in a unique way because of its combination of large field-of-view hard X-ray and gamma-ray detectors. [ABSTRACT FROM AUTHOR]

We review the main features and scientific results obtained during the first 10 years of the AGILE space Mission, a scientific program of the Italian Space Agency (ASI) focused on high-energy astrophysics. The Mission was proposed in 1997 for the ASI "Small Mission" Program and the instrument has been developed with the programmatic and technical support of INAF and INFN. The AGILE satellite was launched on April 23, 2007, from the Indian base in Sriharikota. We describe first the characteristics of the instrument and of the satellite. AGILE is a very innovative space Mission for its class, and is capable of reaching an optimal scientific performance for the study of astrophysical sources with detectors operating in the energy ranges 20–60 keV, 400 keV–100 MeV, 50 MeV–30 GeV. We review the main scientific achievements of the Mission with contributions to the study of the most energetic Galactic and extragalactic sources including accreting compact objects (neutron stars and black holes), pulsars, Supernova Remnants, Active Galactic Nuclei (AGN). AGILE made several important discoveries: a novel mechanism of particle acceleration operating in the Crab Nebula, the emission of transient gamma rays at the moment of launching of relativistic jets from Galactic compact sources, the first evidence of hadronic cosmic ray production from Galactic sources, the emission of very powerful gamma-ray flares from accreting super-massive black holes in AGN, sudden spectral transitions in gamma-ray bursts. Furthermore, AGILE is now providing unique contributions for the study of gravitational wave source counterparts and cosmic neutrino emitters. Also terrestrial applications are relevant for the science program: AGILE discovered gamma-ray emission above 20 MeV from Terrestrial gamma-ray flashes produced by powerful lighting and thunderstorms. This discovery has an important impact for environmental and atmospheric studies. In addition, particle resonance effects and time-dependent precipitations in the lower Earth magnetosphere can by addressed by AGILE studying the charged particles detected by the instrument. The AGILE satellite is operating nominally, and continues to observe the sky searching for the most energetic phenomena of our Universe. [ABSTRACT FROM AUTHOR]

AGILE has been coordinating multi-wavelength campaigns on active galactic nuclei with several major observing facilities since its launch in 2007. This effort allowed us to investigate some remarkable sources both on short and on long time-scales, deriving information on the physical mechanisms responsible for the emission in different energy bands. A complete review of the whole set of AGILE results on extra-galactic sources and their theoretical interpretation is well beyond the scope of this paper, therefore I will present an overview on an handful of outstanding objects and describe the most recent observations. [ABSTRACT FROM AUTHOR]

Gamma-ray emitting binaries (GREBs) are complex systems. Its study became in the last years a major endeavour for the high-energy astrophysics community, both from an observational and a theoretical perspective. Whereas the accumulation of observation time for most Galactic gamma-ray sources is typically leading to highly accurate descriptions of their steady phenomenology, GREBs keep providing "exceptions to the rule" either through long-term monitoring of known systems or in the discovery of new sources of this class. Moreover, many GREBs have been identified as powerful radio, optical and X-ray emitters, and may significantly contribute as well to the Galactic cosmic-ray sea. Their understanding implies, therefore, solving a puzzle in a broad-band and multi-messenger context. In these proceedings we will summarise our current understanding of GREBs, emphasising the most relevant observational results and reviewing a number of controversial properties. [ABSTRACT FROM AUTHOR]

We present the results of a decade of X‐ray observations of the gamma ray loud binary HESS J0632+057 and interpret the available broadband data in view of the system geometry and emission mechanisms. We have performed an analysis of all X‐ray data available to date from Swift, XMM‐Newton, Chandra, NuSTAR, and Suzaku. We refine the orbital period of the system to be 316.8−1.4+2.6 days (95% c.l.), consistent with previous studies but measured with significantly better accuracy. We report on a hydrogen column density and spectral slope variation along the orbit. We argue that the observed variability can be explained within an "inclined disk" model in which the orbit of the compact object is inclined to the disk of the Be star. We show that the observed X‐ray to TeV emissions can originate from a broken cut‐off power‐law population of electrons and describe a way in which future X‐ray/TeV observations can distinguish between the proposed model and the alternative flip‐flop emission scenario of this system. [ABSTRACT FROM AUTHOR]

η Carinae is composed of two very massive stars orbiting each other in 5.5 years. The primary star features the densest known stellar wind, colliding with that expelled by its companion. The wind collision region dissipates energy and accelerates particles up to relativistic energies, producing nonthermal X‐ray and γ‐ray emission detected by Beppo‐SAX, INTEGRAL, Swift, Suzaku, Agile, Fermi, and H.E.S.S. The orbital variability of the system provides key diagnosis of the physics involved and of the emission mechanisms. The low‐energy component, which cuts off below 10 GeV and varies by a factor <2 along the orbit, is likely of the inverse Compton origin. The high‐energy component varies by larger factors and differently during the two periastrons observed by Fermi. These variations match the predictions of simulations assuming a magnetic field in the range 0.4–1 kG at the surface of the primary star. The high‐energy component and the thermal X‐ray emission were weaker than expected around the 2014 periastron suggesting a modification of the inner wind density. Diffuse shock acceleration in the complex geometry of the wind collision zone provides a convincing match to the observations and new diagnostic tools to probe the geometry and energetics of the system. A future instrument sensitive in the MeV energy range could discriminate between lepto‐hadronic and hadronic models for the gamma‐ray emission. At higher energies, the Cherenkov Telescope Array will distinguish orbital modulations of the high‐energy component from those of ultraviolet‐TeV photoabsorption providing a wealth of information constraining acceleration physics under more extreme conditions than found in supernova remnants (SNR). [ABSTRACT FROM AUTHOR]

Interactions between the extragalactic background light (EBL) and very high-energy (VHE; |$E \gt 10\, \rm GeV$|⁠) γ-rays from cosmological sources alter their γ-ray spectrum. The stronger absorption of harder γ-rays causes a steepening of the observed γ-ray spectrum at the high-energy tail, which can be expressed by an increase of the power index. The effect provides a link between high-energy astrophysics and the evolution of galaxies. In this work we develop a new hybrid EBL model by augmenting our previous analytic model with information from semi-analytic galaxy catalogues. The model allows us to study the γ-ray opacities of individual γ-ray paths through the (simulated) universe and evaluate the effect of local fluctuations in the EBL intensity along each path. We confirm an order of magnitude fluctuations in the local EBL (based on the cumulative light from model galaxy distributions within spheres of |$50\, h^{-1}\, \rm Mpc$|⁠). However, the effect of these fluctuations on the VHE γ-ray opacity is insignificant due to the overall very small contribution of the local EBL to the total EBL. We also investigate the effect that a galaxy crossing the line of sight of the γ-ray source may have on the VHE spectrum. We find that galaxies with stellar masses of |$M \gt 10^{11}\, \rm M_{\odot}$| could have a significant effect on the γ-ray absorption. It is unlikely that the observed variation in the spectral index is caused by the proximity of single galaxies to the γ-ray paths as these are extremely rare. [ABSTRACT FROM AUTHOR]

The improvement on the Imaging Air Cherenkov Technique (IACT) led to the discovery of a new type of sources that can emit at very high energies: the gamma-ray binaries. Only six systems are part of this exclusive class. We summarize the latest results from the observations performed with the MAGIC telescopes on different systems as the gamma-ray binary LS I + 6 1 ∘ 303 and the microquasars SS 433, V404 Cygni and Cygnus X-1, which are considered potential VHE gamma-ray emitters. The binary system LS I + 6 1 ∘ 303 has been observed by MAGIC in a long-term monitoring campaign. We show the newest results of our search for super-orbital variability also in context of contemporaneous optical observations. Besides, we will present the results of the only super-critical accretor known in our galaxy: SS 433. We will introduce the VHE results achieved with MAGIC after 100 h of observations on the microquasar Cygnus X-1 and report on the microquasar V404 Cyg, which has been observed with MAGIC after it went through a series of exceptional X-ray outbursts in June 2015. [ABSTRACT FROM AUTHOR]

Since the launch of the Fermi Gamma-ray Space Telescope in 2008, the onboard Large Area Telescope (LAT) has detected gamma-ray pulsations from more than 200 pulsars. A large fraction of these remain undetected in radio observations, and could only be found by directly searching the LAT data for pulsations. However, the sensitivity of such "blind" searches is limited by the sparse photon data and vast computational requirements. In this contribution we present the latest large-scale blind-search survey for gamma-ray pulsars, which ran on the distributed volunteer computing system, Einstein@Home , and discovered 19 new gamma-ray pulsars. We explain how recent improvements to search techniques and LAT data reconstruction have boosted the sensitivity of blind searches, and present highlights from the survey's discoveries. These include: two glitching pulsars; the youngest known radio-quiet gamma-ray pulsar; and two isolated millisecond pulsars (MSPs), one of which is the only known radio-quiet rotationally powered MSP. [ABSTRACT FROM AUTHOR]

This paper details on the discovery of 21 pulsars using the Giant Metrewave Radio Telescope (GMRT) from targeted (Fermi directed search) and blind surveys (GMRT High Resolution Southern Sky - GHRSS) and results from the follow up studies. We discovered seven millisecond pulsars (MSPs) in the Fermi directed searches, which are the first Galactic MSPs discovered with the GMRT. We have discovered 13 pulsars (including a MSP and two mildly recycled pulsars) with the GHRSS survey, which is an off-Galactic-plane survey at 322 MHz with complementary target sky (declination range −40 deg to −54 deg) to other ongoing low-frequency surveys by GBT and LOFAR. The simultaneous time-domain and imaging study for localising pulsars and transients and efficient candidate investigation with machine learning are some of the features of the GHRSS survey, which are also finding application in the SKA design methodology. [ABSTRACT FROM AUTHOR]

A radio polarization study of gamma-ray-detected pulsars reveals a surprising tendency for the magnetic and rotation axes to be relatively aligned. This provides tension with gamma-ray models, which disfavour such alignment. The lack of correlation between these findings and those derived from the gamma-ray light curves suggests problems in the models. To make the data consistent with a random orientation of the magnetic field the emission regions could be assumed to extend outside what is traditionally thought to be the open-field-line region in a magnetic inclination angle dependent way. Both acceptance and rejection of this hypothesis has important consequences. Finally, a unification scheme is proposed to explain the observational differences between gamma-ray loud and gamma-ray quiet radio pulsars. This unification scheme takes the orientation of the line of sight and the magnetic inclination angle to be key parameters affecting both the radio and gamma-ray light-curve morphology. [ABSTRACT FROM AUTHOR]

The Cosmic Ray (CR) physics has entered a new era driven by high precision measurements coming from direct detection (especially AMS-02 and PAMELA) and also from gamma-ray observations (Fermi-LAT). In this review we focus our attention on how such data impact the understanding of the supernova remnant paradigm for the origin of CRs. In particular we discuss advancement in the field concerning the three main stages of the CR life: the acceleration process, the escape from the sources and the propagation throughout the Galaxy. We show how the new data reveal a phenomenology richest than previously thought that could even challenge the current understanding of CR origin. [ABSTRACT FROM PUBLISHER]

It is hypothesized that some young supernovae might have the correct properties to accelerate cosmic rays, which in turn might generate gamma-ray emission by-products. We search for gamma-ray excesses towards supernovae in nearby galaxies which were serendipitously within the field of view of the HESS telescopes within a year of the supernova event. HESS cherenkov air-shower data collected between December 2003 and March 2015 were considered and compared to recent catalogs. Nine candidate supernovae were identified and analysed. No significant emission from these supernovae has been found, and upper limits for their very high energy emission are reported. [ABSTRACT FROM PUBLISHER]

e-ASTROGAM is a gamma-ray observatory operating in a broad energy range, 0.15 MeV – 3 GeV, recently proposed as the M5 Medium-size mission of the European Space Agency. It has the potential to revolutionize the astronomy of medium-energy gamma-rays by increasing the number of known sources in this domain by more than an order of magnitude and providing gamma-ray polarization information for many of these sources. In these proceedings, we discuss the expected capacity of the mission to study the physics of supernovae, both thermonuclear and core-collapse, as well as the origin of cosmic rays in SN shocks. [ABSTRACT FROM PUBLISHER]

Gamma ray lines are expected to be emitted as part of the afterglow of supernova explosions, because radioactive decay of freshly synthesised nuclei occurs. Significant radioactive gamma ray line emission is expected from 56Ni and 44Ti decay on time scales of the initial explosion (56Ni, τ ~days) and the young supernova remnant (44Ti,τ ~90 years). Less specific, and rather informative for the supernova population as a whole, are lessons from longer lived isotopes such as 26Al and 60Fe. From isotopes of elements heavier than iron group elements, any interesting gamma-ray line emission is too faint to be observable. Measurements with space-based gamma-ray telescopes have obtained interesting gamma ray line emissions from two core collapse events, Cas A and SN1987A, and one thermonuclear event, SN2014J. We discuss INTEGRAL data from all above isotopes, including all line and continuum signatures from these two objects, and the surveys for more supernovae, that have been performed by gamma ray spectrometry. Our objective here is to illustrate what can be learned from gamma-ray line emission properties about the explosions and their astrophysics. [ABSTRACT FROM PUBLISHER]

Gamma-ray observations for Supernova remnant (SNR)-molecular cloud (MC) association systems play an important role in the research on the acceleration and propagation of cosmic-ray protons. Through the analysis of 5.6 years of Fermi-Large Area Telescope observation data, here we report on the detection of a gamma-ray emission source near the SNR Kesteven 41 with a significance of 24σ in 0.2–300 GeV. The best-fit location of the gamma-ray source is consistent with the MC with which the SNR interacts. Several hypotheses including both leptonic and hadronic scenarios are considered to investigate the origin of these gamma-rays. The gamma-ray emission can be naturally explained by the decay of neutral pions produced via the collision between high energy protons accelerated by the shock of Kesteven 41 and the adjacent MC. The electron energy budget would be too high for the SNR if the gamma-rays were produced via inverse Compton (IC) scattering off the Cosmic Microwave Background (CMB) photons. [ABSTRACT FROM PUBLISHER]

Recent very high energy observations of the galactic centre region performed by H.E.S.S. revealed the presence of a powerful PeVatron. This is the first of such objects detected, and its most plausible counterpart seems to be associated to Sgr A*, the supermassive black hole in the centre of our galaxy. The implications of this discovery will be discussed, in particular in the context of the problem of the origin of galactic cosmic rays. [ABSTRACT FROM PUBLISHER]

We report a significant hardening of the Fermi-LAT gamma-ray spectrum from the core of Cen A at E > 2.4 GeV, suggesting there is a source of high energy particles in the core of Cen A which is in addition to the jet component. We show that the observed gamma-ray spectrum is compatible with either a spike in the dark matter halo profile or a population of millisecond pulsars. This work gives a strong indication of new gamma-ray production mechanisms in active galactic nuclei and could even provide evidence for the clustering of heavy dark matter particles around black holes. [ABSTRACT FROM PUBLISHER]

We review the current observational and theoretical status of the polarization measurements of Gamma-ray Bursts at all wavelengths. Gamma-Ray Bursts are thought to be produced by an ultra-relativistic jet, possibly powered by a black hole. One of the most important open point is the composition of the jet: the energy may be carried out from the central source either as kinetic energy (of baryons and/or pairs), or in electromagnetic form (Poynting flux). The polarization properties are expected to help disentangling main energy carrier. The prompt emission and afterglow polarization are also a powerful diagnostic of the jet geometry. [ABSTRACT FROM PUBLISHER]

Shocks that arise from baryonic in-fall and merger events during the structure formation are believed to be a source of cosmic rays. These "structure formation cosmic rays" (SFCRs) would essentially be primordial in composition, namely, mostly made of protons and alpha particles. However, very little is known about this population of cosmic rays. One way to test the level of its presence is to look at the products of hadronic reactions between SFCRs and the ISM. A perfect probe of these reactions would be Li. The rare isotope Li is produced only by cosmic rays, dominantly in αα → 6Li fusion reactions with the ISM helium. Consequently, this nuclide provides a unique diagnostic of the history of cosmic rays. Exactly because of this unique property is Li affected most by the presence of an additional cosmic ray population. In turn, this could have profound consequences for the Big-Bang nucleosynthesis: cosmic rays created during cosmic structure formation would lead to pre-Galactic Li production, which would act as a "contaminant" to the primordial 7Li content of metalpoor halo stars. Given the already existing problem of establishing the concordance between Li observed in halo stars and primordial 7Li as predicted by the WMAP, it is crucial to set limits to the level of this "contamination". However, the history of SFCRs is not very well known. Thus we propose a few model-independent ways of testing the SFCR species and their history, as well as the existing lithium problem: 1) we establish the connection between gamma-ray and Li production, which enables us to place constraints on the SFCR-made lithium by using the observed Extragalactic Gamma-Ray Background (EGRB); 2) we propose a new site for testing the primordial and SFCR-made lithium, namely, low-metalicity High-Velocity Clouds (HVCs), which retain the pre-Galactic composition without any significant depletion. Although using one method alone may not give us strong constraints, using them in concert will shed a new light on the SFCR population and possibly give some answers about the pressing lithium problem. .

Compact Symmetric Objects (CSO) are considered to be the young version of Fanaroff-Riley type I and type II radio galaxies, with typical sizes smaller than 1 kpc and ages of the order of a few thousand years. Before the launch of the Fermi satellite, young radio sources were predicted to emerge as a possible new y-ray emitting population detectable by the Large Area Telescope (LAT). After more than 6 years of Fermi operation, the question of young radio sources as y-ray emitting objects still remains open. In this contribution, we discuss candidate y-ray emitting CSO and future perspective for detecting young radio sources with Fermi -LAT. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

During the last decades, increasingly precise astronomical observations of the Galactic Centre (GC) region at radio, infrared, and X-ray wavelengths laid the foundations to a detailed understanding of the high energy astroparticle physics of this most remarkable location in the Galaxy. Recently, observations of this region in high energy (HE, 10 MeV–100 GeV) and very high energy (VHE, > 100 GeV) γ -rays added important insights to the emerging picture of the Galactic nucleus as a most violent and active region where acceleration of particles to very high energies – possibly up to a PeV – and their transport can be studied in great detail. Moreover, the inner Galaxy is believed to host large concentrations of dark matter (DM), and is therefore one of the prime targets for the indirect search for γ -rays from annihilating or decaying dark matter particles. In this article, the current understanding of the γ -ray emission emanating from the GC is summarised and the results of recent DM searches in HE and VHE γ -rays are reviewed. [ABSTRACT FROM AUTHOR]

SN2014J is the closest supernova of type Ia that occurred in the last 40 years. This provides an opportunity for unprecedented observational detail and coverage in many astronomical bands, which will help to better understand the still unknown astro-physics of these supernovae. For the first time, such an event occurs sufficiently nearby so that also γ-rays are able to contribute to such investigations. This is important, as the primary source of the supernova light is the radioactive energy from about 0.5 M⊙ of 56Ni produced in the explosion, and the γ rays associated with this decay make the supernova shine for months. The INTEGRAL γ-ray observatory of ESA has followed the supernova emission for almost 5 months. The characteristic γ-ray lines from the 56Ni decay chain through 56Co to 56Fe have been measured. We discuss these observations, and the implications of the measured γ-ray line characteristics as they evolve. (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]