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Quasi-simultaneous VLBI observations at 15-86GHz have shown that in the classical superluminal radio source 3C273, the spectral index (S / ) has a systematic variation along the jet. For epoch 1995.15, a spectral reversal was observed at core distance 1.5mas, where the superluminal knot C12 located. Similarly, for epoch 1997.18, two spectral reversals were observed at core distances of 1.8mas and 4.2mas, where superluminal knots C11 and C14 were, respectively. These spectral reversals are associated with local maxima of the jet width. We suggest that this phenomenon may be related to a stratification of the jet structure, i.e., its physical parameters (flow velocity, Doppler factor, electron density and energy, magnetic field strength, etc.) are substantially dependent on the distance from the jet axis. These properties may be naturally formed through gasdynamic processes when the jet expands into a lower pressure ambient medium.

3C 273 has been observed with VLBI for more than 30 years. The entire data have shown that the position angle of the superluminal knots ejected from the core varies periodically with a period of ~15 years. Moreover, their apparent velocity observed during the period 1963 – 1997 has systematically decreased by a ≳ 2. These remarkable properties are explained in terms of a precessing factor of ~ jet model, in which the ejection Lorentz factor of the superluminal knots has been decreasing during the last thirty years and has superposed on it a short-term (~5 year) oscillation. The periodic variations derived by the model-fitting are compared with the variations in the optical flux density. Binary black hole models are briefly discussed to show possible relations of the observed periods to the periods involved in a binary system (orbital motion, spin of black hole, accretion-disk rotation and Newtonian-driven precession etc.).

The intraday variations (IDV hereafter) of the total and the polarized flux density observed at 20 cm in 0917+624 (in May 1989) are analysed. It is shown that the IDV can be interpreted in terms of refractive interstellar scintillation. The observed variations are dominated by one scintillating component with a timescale $\tau\approx0.6$ days and a scintillation index $m\approx0.04$. In addition, one more scintillating component with a shorter timescale and a smaller scintillation index is needed to improve the fit to the observed Q - and U -light curves. The relationship between the scintillating components and the VLBI components are also discussed. It is shown that Doppler beaming of the 20 cm scintillating components with a Lorentz factor $\stackrel {> }{_\sim}$ 7 may be needed to reduce their intrinsic brightness temperatures significantly below the inverse-Compton limit.

Intraday variability in compact flat-spectrum radio sources has been intensively studied in recent years. For most IDV events the apparent brightness temperatures derived from the observed timescales are in the range of Tb,app ~ 1016−18 K. For extremely rapid variations, Tb,app can reach up to ~ 1021 K (e.g. Kedziora-Chudczer et al., 1997). Refractive interstellar scintillation may be the most likely extrinsic mechanism (Rickett et al., 1995; Qian, 1994a; Qian, 1994b). Especially for the case of extreme Tb,app (> 1018 K) RISS may be dominant (Dennet-Thorpe and de Bruyn, 2000). However, some IDV events with Tapp ~ 1017−18 K show evidence for an intrinsic origin e.g. the correlated radio-optical intraday variations observed in the BL Lac object 0716+714 (Wagner and Witzel, 1995, Qian et al, 1996). It seems important to distinguish between IDV which is a phenomenon intrinsic to the compact radio sources and IDV which is primarily due to RISS. Multifrequency polarization and VLB I observations would be most helpful (Gabuzda and Kochanev, 1997).

The quasar 3C345 is one of the best examples of extra-galactic jet with superluminal components moving on helical paths. To get better constraints on the jet physics, we have imaged the source at the highest possible resolution, using mm-VLBI and space-VLBI observations. We observed 3C 345 at 5 different epochs so far (1 epoch with 3-mm ground VLBI, and 4 epochs with space VLBI at 6cm and 18 cm). These measurements expand significantly our ongoing monitoring of 3C 345. In this contribution, we discuss the properties of the jet and its moving components in the immediate vicinity of the core (< 3 mas).

We present three epochs of VSOP observations of the BL Lac object2007+777 at 5 GHz. Compared with the ground-based VLBA data, the space baselines with HALCA clearly reveal a more detailed and finer source structure. Mainly based on the quite uniform and circular UV-coverages of the VLBA, and using a new cross-self calibration method, we have found evidence for weak structural changes on a timescale of two weeks in the core region of this intraday variable source. The physical causes for these variations are discussed.

The Fermi LAT detected an increase in γ-ray activity of the quasar 0836+710 (z=2.17) in Spring 2011 that culminated in a sharp γ-ray flare at the end of 2011 when the source reached a flux of 2.9×10−6 phot s−1cm−2 at 0.1-200 GeV. We monitor the quasar at optical wavelengths in photometric and polarimetric modes, at millimeter and centimeter wavelengths, and with the VLBA at 43 GHz. The optical brightness of the quasar increased by ~0.5 mag in R band and the degree of polarization oscillated between ~1% and ~6% during the highest γ-ray state, while the position angle of polarization rotated by ~300°. We have identified in the VLBA images a strong, highly polarized component that moves with an apparent speed of ~20 c. The component emerged from the core in the beginning of the γ-ray event and reached a flux maximum at the peak of the γ-ray outburst. We present the results of a correlative analysis of variations at different wavelengths along with the kinematic parameters of the parsec scale jet. We discuss the location of the high γ-ray emission in the relativistic jet, as well as the emission mechanisms responsible for γ-ray production.

We present results from the first observing epoch (December 1997) of 5 GHz VSOP monitoring program of the parsec-scale jet in 3C 273. At the resolution achieved by the VSOP observation, the jet is transversely resolved, and it exhibits significant edge brightening. The latter may result from velocity gradients or reacceleration zones existing in the jet, due to interactions between the faster jet plasma and slower, non-relativistic outflow or ambient medium.

We present the data of three observing campaigns with the VLA performed to study intraday variability in compact extragalactic radio sources. The first campaign (May 1989) lasted 5 days; total intensity as well as linear polarization data were obtained at 2, 3.6, 6, and 20 cm wavelength for 9 program sources. In the second observing run (February 1990), a five-antenna subarray of the VLA was used for 25 days to monitor the sources 0716+714, 0917+624, and 0954+658, with dense sampling at the same four wavelengths. In the third run (October 1992), which lasted 22 days, 8 program sources were monitored at 3.6, 6, and 20 cm wavelength, and 2 additional sources at 6 cm only, again with a five-antenna subarray. We present summary tables, light curves, and structure functions from these data sets.

We monitored the superluminal QSO 3C 345 at three epochs during a one-year period in 1995--1996, observing with the VLBA at 22, 15, 8.4, and 5 GHz. We imaged the radio source both in total and in polarized intensity. In the images at 5 and 8.4 GHz, the jet emission is traced up to 20 milliarcseconds (mas) from the jet core. In the 15 and 22 GHz images, we identify several enhanced emission regions moving at apparent speeds of 5c. Images of the linear polarized emission show predominantly an alignment of the electric vector with the extremely curved jet along the inner part of the high frequency jet. At 5 GHz, the jet shows remarkably strong fractional polarization (m~15%) with the electric vector perpendicular to the jet orientation., LaTeX file, 6 pages, 2 figures, needs "elsart" style package To be published in New Astronomy Reviews, special issue: Proceedings of the 4th EVN/JIVE VLBI Symposium, Eds. Garrett, M.A., Campbell, R.M., & Gurvits, L.J

The VLBA has been used over a period of four years to study the internal motions within a sample of quasars and AGN. In most sources, features appear to propagate away from the central engine along a well collimated radio jet with apparent transverse velocities between zero and 10c, with some evidence for apparent accelerations and decelerations. The distribution of apparent velocity is not consistent with any simple ballistic model and appears to require either a spread in intrinsic velocity or a difference between the bulk velocity and pattern velocity. The dependence of apparent angular velocity with redshift is consistent with standard Friedmann world models. Further observations of a larger source sample, especially at large redshift may lead to meaningful constraints on world models.

Resolution improvements achieved with the ongoing space VLBI mission VSOP provide an excellent opportunity for studying compact, parsec-scale jets in active galactic nuclei. Visibilities on space baselines contain structural information that cannot be recovered by super-resolving ground-array data at the same frequency. We illustrate this by discussing the results of recent VSOP observations of parsec-scale jets in 0836+710 and 3C273.

We present deep 8.4 GHz VLBI images of the faint nuclei in seven lobe-dominated quasars (LDQs): 3C 175, 3C 204, 3C 205, 3C 215, 3C 351, 0821)447, and 0839)616 . These images, made from data obtained in 1988 and 1991 with high-sensitivity VLBI arrays, have a typical rms noise level of D0.03 mJy per beam and resolution of D0.5 mas. The parsec-scale jet structures are generally greater in extent and complexity than those seen on many previous, less sensitive maps of other LDQ nuclei; the jet in 3C 204 bends through at least 10i, and the jet in 3C 205 extends out to D5 mas. Observations of 3C 204, 3C 205, and 0839)616 at two epochs reveal clear —ux density variability in 3C 204 and 3C 205, but none of these sources shows any evidence for outward motion of jet components; their mean appar- ent transverse velocity is ((0.47 ^ 0.43)h~1c, from which we determine a 3 p upper limit on their mean expansion speed of D0.8 h~1c km s~1 Mpc~1, The innermost jet knot in 3C 204 (H 0 \ 100 hq 0 \ 0.5). underwent a marginal position angle swing away from the mean jet axis. We also present the —rst 8.4 GHz A-array VLA images for all the objects except 3C 351. All the sources except 0821)447 display one-sided VLBI and VLA jets, which lie on the same side of the VLBI core and are generally well aligned (to within D5i). In addition, we analyze the statistics of projected linear size, relative nuclear strength, and misalignment angle for the 3CR and Jodrell Bank LDQ samples. We conclude that our results are generally consistent with a fairly simple relativistic jet model and with the uni—cation of core- dominated quasars, lobe-dominated quasars, and possibly even powerful radio galaxies; there is a mar- ginal suggestion that the quasars in our samples have a restricted range of angles to the line of sight (e.g., 0i to D45i). Subject headings: galaxies: jetsgalaxies: structurequasars: generalradio continuum: galaxies ¨ techniques: interferometric

We have used the VLBA at 15 GHz to image the structure of 132 strong compact AGN and quasars with a resolution better than one milliarcsecond and a dynamic range typically exceeding 1000 to 1. These observations were made as part of a program to investigate the sub-parsec structure of quasars and AGN and to study the changes in their structure with time. Many of the sources included in our study, particularly those located south of +35 degrees, have not been previously imaged with milliarcsecond resolution. Each of the sources has been observed at multiple epochs. In this paper we show images of each of the 132 sources which we have observed. For each source we present data at the epoch which had the best quality data. The milliarcsecond jets generally appear one-sided but two-sided structure is often found in lower luminosity radio galaxies and in high luminosity quasars with gigahertz peaked spectra. Usually the structure is unresolved along the direction perpendicular to the jet, but a few sources have broad plumes. In some low luminosity radio galaxies, the structure appears more symmetric at 2 cm than at long wavelengths. The apparent long wavelength symmetry in these sources is probably due to absorption by intervening material. A few sources contain only a single component with any secondary feature at least a thousand times weaker. We find no obvious correlation of radio morphology and the detection of gamma-ray emission by EGRET.

We report results on the QSO 3C345 obtained from VLBA observations carried out on epochs 1995.84, 1996.41, and 1996.81, at 22, 15, 8.4 and 5 GHz. Total and polarized images VLBI images were obtained. We observed superluminal motions of components C8 and C7 with respect to the core component D. We imaged also the remarkably complex polarization structure near the core.

▪ Abstract Observations of parsec-scale radio jets associated with active galactic nuclei are reviewed, with a particular emphasis on high-luminosity core-dominated sources where the most detailed information on individual objects exists. Extensive imaging surveys with very long baseline interferometry (VLBI) have made possible a morphologic classification of compact radio sources and systematic studies of the statistics of apparent (often faster-than-light) motions found in an increasing number of sources. VLBI monitoring studies at centimeter and millimeter wavelengths, enhanced by spectral and polarization imaging, of representative types of AGN can discriminate detailed physical models. The observations are especially discussed in light of the variant of the relativistic beaming model that explains kinematic, spectral, and polarization properties of parsec-scale jets through shocks in an underlying continuous jet flow.

We present the results of the -rst systematic study of correlated variability in the X-ray emission and the parsec-scale radio structure of the "" superluminal II quasar 3C 345. This quasar is one of a class of core-dominated Nat-spectrum radio sources that are believed to emit X-rays via the synchrotron self- Compton process. By studying its variability in X-rays, we can test this hypothesis for 3C 345 by model- ing the expected inverse-Compton Nux from the parsec-scale jet, using parameters derived from multifrequency VLBI imaging. Since the predicted X-ray Nux is very sensitive to the physical parameters of the compact radio-emitting regions, a study of variability, with quasi-simultaneous X-ray and VLBI data, is of particular interest because it is less sensitive to assumptions in the adopted model. The soft X-ray Nux density of 3C 345 was observed by the ROSAT PSPC instrument at seven epochs during 1990E1993, during which time its Nux density varied by a factor of 2, but with no change in spectral index. The X-ray points closely track the high-frequency radio Nux light curve. Using a series of VLBI images, we followed the time evolution of spectral shapes and angular sizes of the nucleus and the strongest jet component "" C7,II at a distance of ^0.5 mas (2 pc) from the nucleus, and derived physical parameters for the components. We -nd that component C7, not the nucleus, is the dominant generator of the observed X-rays during 1992E1993. For the nucleus, we applied the inhomogeneous-jet model of Koe nigl and found -nd that it underpredicts the X-ray Nux for any plausible combination of physical parameters derived from observation. A homogeneous sphere, with Nux density peaking at a few GHz, is adequate for modeling the evolution of C7. This sphere model requires that C7 dominated the X-ray emission, unless its Doppler factor is ?10. This contrasts with the situation at epoch 1990.55, for which comparable data (Unwin et al. 1994) showed that the nucleus and C5 (the dominant jet feature at that time) both contributed to the X-ray emission. We discuss the lack of detectable c-rays from 3C 345 in relation to other c-ray loud blazars with which it shares many observational properties. Combining the superluminal speed (from VLBI) and Doppler factor deduced from the synchrotron self-Compton calcu- lation, we solve for the jet kinematics at the position of C7, and we -nd that the jet bends away from the line of sight (from h ^ 2i to ^10i) and accelerates from c ^ 5 to over the range of (deprojected)

We present some of the results from mapping the turnover frequency, turnover flux and magnetic field distributions in the parsecscale jets of 3C 273 and 3C 345. We discuss the existing problems of VLBI image analysis related to the extraction of spectral information.

LOFAR is a new digital radio interferometer that is being build in The Netherlands. By sampling the radio waves with fast ADCs it can digitally store the whole waveform information and analyze transient events like air showers after they have been recorded. To demonstrate its ability to measure air showers we are building LOPES (a LOFAR PrototypeStation) at the site of an existing air shower array (KASCADE-Grande). The first phase consisting of 10 antennas is already running. It has demonstrated how digital interference suppression and beamforming can overcome the problem of radio interference and pick out air shower events.

We report on the first detection of the theoretically-predicted rare phenomenon of multiple parsec-scale imaging of an active galactic nucleus induced by refractive effects due to localized foreground electron density enhancements, e.g., in an AU-scale plasma lens(es) in the ionized component of the Galactic interstellar medium. We detected multiple imaging in the low galactic latitude (b=-2 deg) quasar 2023+335 from the 15.4 GHz MOJAVE observations when the source was undergoing an ESE. While the parsec-scale jet of the source normally extends along PA -20 deg, in the 28 May 2009 and 23 July 2009 images a highly significant multi-component pattern of secondary images is stretched out nearly along the constant galactic latitude line with a local PA 40 deg, indicating that the direction of relative motion of the plasma lens is close to orbital. Weaker but still detectable imaging patterns at similar position angles are sporadically manifest at several other epochs. Modeling the ESE that occurred in early 2009 and lasted ~0.14 yr, we determined that the foreground screen has a double-lens structure, with proper motion (~6.8 mas/yr), and angular size (~0.27 mas). We also found that the angular separation between the two brightest sub-images roughly follows a wavelength-squared dependence expected from plasma scattering. Furthermore, by analyzing archival non-simultaneous VLBA observations covering a wide frequency range from 1.4 to 86 GHz, we found that the scattered angular size of the VLBI core follows a $\nu^{-1.89}$ dependence, implying the presence of a turbulent, refractive dominated scattering screen that has a confined structure or is truncated transverse to the line of sight toward 2023+335., Comment: 13 pages, 7 figures, accepted for publication in A&A on 22 May, 2013

Relativistic, charged particles present in extensive air showers (EAS) lead to a coherent emission of radio pulses which are measured to identify the shower initiating high-energy cosmic rays. Especially during thunderstorms, there are additional strong electric fields in the atmosphere, which can lead to further multiplication and acceleration of the charged particles and thus have influence on the form and strength of the radio emission. For a reliable energy reconstruction of the primary cosmic ray by means of the measured radio signal it is very important to understand how electric fields affect the radio emission. In addition, lightning strikes are a prominent source of broadband radio emissions that are visible over very long distances. This, on the one hand, causes difficulties in the detection of the much lower signal of the air shower. On the other hand the recorded signals can be used to study features of the lightning development. The detection of cosmic rays via the radio emission and the influence of strong electric fields on this detection technique is investigated with the LOPES experiment in Karlsruhe, Germany. The important question if a lightning is initiated by the high electron density given at the maximum of a high-energy cosmic-ray air shower is also investigated, but could not be answered by LOPES. But, these investigations exhibit the capabilities of EAS radio antenna arrays for lightning studies. We report about the studies of LOPES measured radio signals of air showers taken during thunderstorms and give a short outlook to new measurements dedicated to search for correlations of lightning and cosmic rays.

To better understand the radio signal emitted by extensive air-showers and to further develop the radio detection technique of high-energy cosmic rays, the LOPES experiment was reconfigured to LOPES-3D. LOPES-3D is able to measure all three vectorial components of the electric field of radio emission from cosmic ray air showers. The additional measurement of the vertical component ought to increase the reconstruction accuracy of primary cosmic ray parameters like direction and energy, provides an improved sensitivity to inclined showers, and will help to validate simulation of the emission mechanisms in the atmosphere. LOPES-3D will evaluate the feasibility of vectorial measurements for large scale applications. In order to measure all three electric field components directly, a tailor-made antenna type (tripoles) was deployed. The change of the antenna type necessitated new pre-amplifiers and an overall recalibration. The reconfiguration and the recalibration procedure are presented and the operationality of LOPES-3D is demonstrated.

The Russian Academy of Sciences and Federal Space Agency, together with the participation of many international organizations, worked toward the launch of the RadioAstron orbiting space observatory with its onboard 10-m reflector radio telescope from the Baikonur cosmodrome on July 18, 2011. Together with some of the largest ground-based radio telescopes and a set of stations for tracking, collecting, and reducing the data obtained, this space radio telescope forms a multi-antenna ground-space radio interferometer with extremely long baselines, making it possible for the first time to study various objects in the Universe with angular resolutions a million times better than is possible with the human eye. The project is targeted at systematic studies of compact radio-emitting sources and their dynamics. Objects to be studied include supermassive black holes, accretion disks, and relativistic jets in active galactic nuclei, stellar-mass black holes, neutron stars and hypothetical quark stars, regions of formation of stars and planetary systems in our and other galaxies, interplanetary and interstellar plasma, and the gravitational field of the Earth. The results of ground-based and inflight tests of the space radio telescope carried out in both autonomous and ground-space interferometric regimes are reported. The derived characteristics are in agreement with the main requirements of the project. The astrophysical science program has begun., 54 pages, 11 figures; published by Astronomicheskij Zhurnal (in Russian) and Astronomy Reports (in English)

We investigate the radio and gamma-ray variability of the flat spectrum radio quasar PKS 1510-089 in the time range between 2010 November and 2012 January. In this period the source showed an intense activity, with two major gamma-ray flares detected in 2011 July and October. During the latter episode both the gamma-ray and the radio flux density reached their historical peak. Multiwavelength analysis shows a rotation of about 380 deg of the optical polarization angle close in time with the rapid and strong gamma-ray flare in 2011 July. An enhancement of the optical emission and an increase of the fractional polarization both in the optical and in radio bands is observed about three weeks later, close in time with another gamma-ray outburst. On the other hand, after 2011 September a huge radio outburst has been detected, first in the millimeter regime followed with some time delay at centimeter down to decimeter wavelengths. This radio flare is characterized by a rising and a decaying stage, in agreement with the formation of a shock and its evolution, as a consequence of expansion and radiative cooling. If the gamma-ray flare observed in 2011 October is related to this radio outburst, then this strongly indicates that the region responsible for the gamma-ray variability is not within the broad line, but a few parsecs downstream along the jet., Comment: 14 pages, 12 figures, accepted for publication in MNRAS

We report that active galactic nucleus (AGN) jets are causally connected on parsec scales, based on 15 GHz Very Long Baseline Array (VLBA) data from a sample of 133 AGN jets. This result is achieved through a new method for measuring the product of the jet Lorentz factor and the intrinsic opening angle Gamma*theta_j from measured apparent opening angles in flux density limited samples of AGN jets. The Gamma*theta_j parameter is important for jet physics because it is related to the jet-frame sidewise expansion speed and causal connection between the jet edges and its symmetry axis. Most importantly, the standard model of jet production requires that the jet be causally connected with its symmetry axis, implying that Gamma*theta_j < 1. When we apply our method to the MOJAVE flux density limited sample of radio loud objects, we find Gamma*theta_j = 0.2, implying that AGN jets are causally connected. We also find evidence that AGN jets viewed very close to the line of sight effectively have smaller intrinsic opening angles compared with jets viewed more off-axis, which is consistent with Doppler beaming and a fast inner spine/slow outer sheath velocity field. Notably, gamma-ray burst (GRB) jets have a typical Gamma*theta_j that is two orders of magnitude higher, suggesting that different physical mechanisms are at work in GRB jets compared to AGN jets. A useful application of our result is that a jet's beaming parameters can be derived. Assuming Gamma*theta_j is approximately constant in the AGN jet population, an individual jet's Doppler factor and Lorentz factor (and therefore also its viewing angle) can be determined using two observable quantities: apparent jet opening angle and the apparent speed of jet components., 9 pages, 4 figures

LOPES is a digital antenna array for the radio measurement of cosmic-ray air showers at energies around 1017 eV. It is triggered by the KASCADE-Grande air-shower array at the Karlsruhe Institute of Technology (KIT), Germany. Because of an absolute amplitude calibration and a sophisticated data analysis, LOPES can test models for the radio emission to an up-to-now unachieved level, thus improving our understanding of the radio emission mechanisms. Recent REAS simulations of the air-shower radio emission come closer to the measurements than any previously tested simulations. We have determined the radio-reconstruction precision of interesting air-shower parameters by comparing LOPES reconstructions to both REAS simulations and KASCADE-Grande measurements, and present our latest results for the angular resolution, the energy and the Xmax reconstruction based on the radio measurement of about 500 air showers. Although the precision of LOPES is limited by the high level of anthropogenic noise at KIT, it opens a promising perspective for next-generation radio arrays in regions with a lower ambient noise level.

Quasar 3C 279 is known to exhibit episodes of optical polarization angle rotation. We present new, well-sampled optical polarization data for 3C 279 and introduce a method to distinguish between random and deterministic electric vector position angle (EVPA) variations. We observe EVPA rotations in both directions with different amplitudes and find that the EVPA variation shows characteristics of both random and deterministic cases. Our analysis indicates that the EVPA variation is likely dominated by a random process in the low brightness state of the jet and by a deterministic process in the flaring state., Comment: 6 pages, 2 figures, 4 tables, proceeding to appear on EPJ Web of Conferences "The Innermost Regions of Relativistic Jets and their Magnetic Fields (Granada, Spain,10-14 June 2013)"

In the previous years, LOPES emerged as a very successful experiment measuring the radio emission from air showers in the MHz frequency range. In parallel, the theoretical description of radio emission was developed further and REAS became a widely used simulation Monte Carlo code. REAS 3 as well as CoREAS are based on the endpoint formalism, i.e. they calculate the emission of the air-shower without assuming specific emission mechanisms. While REAS 3 is based on histograms derived from CORSIKA simulations, CoREAS is directly implemented into CORSIKA without loss of information due to histogramming of the particle distributions. In contrast to the earlier versions of REAS, the newest version REAS 3.11 and CoREAS take into account a realistic atmospheric refractive index. To improve the understanding of the emission processes and judge the quality of the simulations, we compare their predictions with high-quality events measured by LOPES. We present results concerning the lateral distribution measured with 30 east-west aligned LOPES antennas. Only the simulation codes including the refractive index (REAS 3.11 and CoREAS) are able to reproduce the slope of measured lateral distributions, but REAS 3.0 predicts too steep lateral distributions, and does not predict rising lateral distributions as seen in a few LOPES events. Moreover, REAS 3.11 predicts an absolute amplitude compatible with the LOPES measurements.

Cosmic ray air showers emit radio pulses at MHz frequencies, which can be measured with radio antenna arrays - like LOPES at the Karlsruhe Institute of Technology in Germany. To improve the understanding of the radio emission, we test theoretical descriptions with measured data. The observables used for these tests are the absolute amplitude of the radio signal, and the shape of the radio lateral distribution. We compare lateral distributions of more than 500 LOPES events with two recent and public Monte Carlo simulation codes, REAS 3.11 and CoREAS (v 1.0). The absolute radio amplitudes predicted by REAS 3.11 are in good agreement with the LOPES measurements. The amplitudes predicted by CoREAS are lower by a factor of two, and marginally compatible with the LOPES measurements within the systematic scale uncertainties. In contrast to any previous versions of REAS, REAS 3.11 and CoREAS now reproduce the shape of the measured lateral distributions correctly. This reflects a remarkable progress compared to the situation a few years ago, and it seems that the main processes for the radio emission of air showers are now understood: The emission is mainly due to the geomagnetic deflection of the electrons and positrons in the shower. Less important but not negligible is the Askaryan effect (net charge variation). Moreover, we confirm that the refractive index of the air plays an important role, since it changes the coherence conditions for the emission: Only the new simulations including the refractive index can reproduce rising lateral distributions which we observe in a few LOPES events. Finally, we show that the lateral distribution is sensitive to the energy and the mass of the primary cosmic ray particles., Comment: Attention: results and conclusion partly changed due to updated calibration. See Astroparticle Physics 75 (2016) 72-74; arXiv:1507.07389

The temporal and spatial spectral evolution of the jets of AGN can be studied with multi-frequency, multi-epoch VLBI observations. The combination of both, morphological and spectral parameters can be used to derive source intrinsic physical properties such as the magnetic field and the non-thermal particle density. In the first two papers of this series, we analyzed the single-dish light curves and the VLBI kinematics of the blazar CTA 102 and suggested a shock-shock interaction between a traveling and a standing shock wave as a possible scenario to explain the observed evolution of the component associated to the 2006 flare. In this paper we investigate the core-shift and spectral evolution to test our hypothesis of a shock-shock interaction. We used 8 multi-frequency VLBA observations to analyze the temporal and spatial evolution of the spectral parameters during the flare. We observed CTA 102 between May 2005 and April 2007 using the VLBA at six different frequencies spanning from 2 - 86 GHz. After the calibrated VLBA images were corrected for opacity, we performed a detailed spectral analysis. From the derived values we estimated the magnetic field and the density of the relativistic particles. The detailed analysis of the opacity shift reveals that the position of the jet core is proportional to nu^-1 with some temporal variations. The value suggests possible equipartition between magnetic field energy and particle kinetic energy densities at the most compact regions. From the variation of the physical parameters we deduced that the 2006 flare in CTA 102 is connected to the ejection of a new traveling feature (t=2005.9) and the interaction between this shock wave and a stationary structure around 0.1 mas from the core. The source kinematics together with the spectral and structural variations can be described by helical motions in an over-pressured jet., 35 pages, 46 figures

Aims. The observation of gamma -ray flares from blazar 0836+710 in 2011, following a period of quiescence, offered an opportunity to study correlated activity at different wavelengths for a high-redshift (z=2.218) active galactic nucleus. Methods. Optical and radio monitoring, plus Fermi-LAT gamma-ray monitoring provided 2008-2012 coverage, while Swift offered auxiliary optical, ultraviolet, and X-ray information. Other contemporaneous observations were used to construct a broad-band spectral energy distribution. Results. There is evidence of correlation but not a measurable lag between the optical and gamma-ray flaring emission. On the contrary, there is no clear correlation between radio and gamma-ray activity, indicating radio emission regions that are unrelated to the parts of the jet that produce the gamma-rays. The gamma-ray energy spectrum is unusual in showing a change of shape from a power law to a curved spectrum when going from the quiescent state to the active state., 11 pages, 10 figures, Accepted for publication in A&A

The GeV observations by Fermi-LAT give us the opportunity to characterize the high-energy emission (100 MeV - 300 GeV) variability properties of the BL Lac object S5 0716+714. In this study, we performed flux and spectral analysis of more than 3 year long (August 2008 to April 2012) Fermi-LAT data of the source. During this period, the source exhibits two different modes of flux variability with characteristic timescales of ~75 and ~140 days, respectively. We also notice that the flux variations are characterized by a weak spectral hardening. The GeV spectrum of the source shows a clear deviation from a simple power law, and is better explained by a broken power law. Similar to other bright Fermi blazars, the break energy does not vary with the source flux during the different activity states. We discuss several possible scenarios to explain the observed spectral break., Comment: 21 pages, 10 figures, Accepted for publication in Advances in Space Research journal

The quasar 1156+295 (4C +29.45) is one of the targets in the Urumqi monitoring program which aimed to search for evidence of annual modulation in the timescales of intra-day variable (IDV) sources. The IDV observations of 1156+295 were carried out nearly monthly from October 2007 to October 2009, with the Urumqi 25m radio telescope at 4.8 GHz. The source has shown prominent IDV of total flux density in most observing sessions with variability timescales of, 7 pages, 8 figures, accepted for publication in A&A

Astronomy and Astrophysics 556 (2013): A67 reproduced with permission from Astronomy and Astrophysics, Context. BL Lacertae objects are variable at all energy bands on time scales down to minutes. To construct and interpret their spectral energy distribution (SED), simultaneous broad-band observations are mandatory. Up to now, the number of objects studied during such campaigns is very limited and biased towards high flux states. Aims.We present the results of a dedicated multi-wavelength study of the high-frequency peaked BL Lacertae (HBL) object and known TeV emitter 1ES 2344+514 by means of a pre-organised campaign. Methods.The observations were conducted during simultaneous visibility windows of MAGIC and AGILE in late 2008. The measurements were complemented by Metsähovi, RATAN-600, KVA+Tuorla, Swift and VLBA pointings. Additional coverage was provided by the ongoing long-term F-GAMMA and MOJAVE programs, the OVRO 40-m and CrAO telescopes as well as the Fermi satellite. The obtained SEDs are modelled using a one-zone as well as a self-consistent two-zone synchrotron self-Compton model. Results. 1ES 2344+514 was found at very low flux states in both X-rays and very high energy gamma rays. Variability was detected in the low frequency radio and X-ray bands only, where for the latter a small flare was observed. The X-ray flare was possibly caused by shock acceleration characterised by similar cooling and acceleration time scales. MOJAVE VLBA monitoring reveals a static jet whose components are stable over time scales of eleven years, contrary to previous findings. There appears to be no significant correlation between the 15 GHz and R-band monitoring light curves. The observations presented here constitute the first multi-wavelength campaign on 1ES 2344+514 from radio to VHE energies and one of the few simultaneous SEDs during low activity states. The quasi-simultaneous Fermi-LAT data poses some challenges for SED modelling, but in general the SEDs are described well by both applied models. The resulting parameters are typical for TeV emitting HBLs. Consequently it remains unclear whether a so-called quiescent state was found in this campaign, 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 CPAN CSD2007-00042 and MultiDark CSD2009-00064 projects of the Spanish Consolider-Ingenio 2010 programme, by grant 127740 of the Academy of Finland, by the DFG Cluster of Excellence “Origin and Structure of the Universe”, by the DFG Collaborative Research Centers SFB823/C4 and SFB876/C3, and by the Polish MNiSzW grant 745/N-HESS-MAGIC/2010/0. The AGILE Mission is funded by the Italian Space Agency (ASI), with scientific and programmatic participation by the Italian Institute of Astrophysics (INAF) and the Italian Institute of Nuclear Physics (INFN). Research partially supported through the ASI grants no. I/089/06/2 and I/042/10/0. The Fermi-LAT Collaboration acknowledges support from a number of agencies and institutes for both development and the operation of the LAT as well as scientific data analysis. These include NASA and DOE in the United States, CEA/Irfu and IN2P3/CNRS in France, ASI and INFN in Italy, MEXT, KEK, and JAXA in Japan, and the K. A.Wallenberg Foundation, the Swedish Research Council and the National Space Board in Sweden. Additional support from INAF in Italy and CNES in France for science analysis during the operations phase is also gratefully acknowledged

High-dynamic range imaging and monitoring with Very Long Baseline Interferometry have considerably increased our knowledge of the parsec-scale properties of compact radio sources. I review some of the properties of individual sources in areas where particular progress has been made in the last few years.

VLBI observations of the extremely gamma-bright blazar PKS 0528+134 at 8, 22, 43, and 86 GHz reveal a strongly bent one-sided-core jet structure with at least three moving and two apparently stationary jet components. At the highest observing frequencies the brightest and most compact jet component (the VLBI core) is unresolved with an upper limit to its size of approximately 50 microarcsec corresponding to approximately 0.2 parsec [H0 = 100 km.s-1.Mpc-1 (megaparsec-1), q0 = 0.5, where H0 is Hubble constant and q0 is the deceleration parameter]. Two 86-GHz VLBI observations performed in 1993.3 and 1994.0 reveal a new jet component emerging with superluminal speed from the core. Linear back-extrapolation of its motion yields strong evidence that the ejection of this component is related to an outburst in the millimeter regime and a preceding intense flare of the gamma-flux density observed in early 1993. This and the radio/optical "light curves" and VLBI data for two other sources (S5 0836+710 and 3C 454.3) suggest that the observed gamma-radiation might be Doppler-boosted and perhaps is closely related to the physical processes acting near the "base" of the highly relativistic jets observed in quasars.

M87 is one of the nearest radio galaxies with a central Super-Massive Black Hole (SMBH) and a prominent relativistic jet. Due to its close distance to the observer and the large SMBH mass, the source is one of the best laboratories to obtain strong observational constraints on the theoretical models for the formation and evolution of the AGN jets. In this article, we present preliminary results from our ongoing observational study about the innermost jet of M87 at an ultra-high resolution of ∼50 μ as achieved by the Global Millimeter-Very Long Baseline Interferometry Array (GMVA). The data obtained between 2004 and 2015 clearly show limb-brightened jets at extreme resolution and sensitivity. Our preliminary analysis reveals that the innermost jet expands in an edge-brightened cone structure (parabolic shape) but with the jet expansion profile slightly different from the outer regions of the jet. Brightness temperatures of the Very Long Baseline Interferometry (VLBI) core obtained from cm- to mm-wavelengths show a systematic evolution, which can be interpreted as the evolution as a function of distance from the BH. We also adopt an alternative imaging algorithm, Bi-Spectrum Maximum Entropy Method (BSMEM), to test reliable imaging at higher angular resolution than provided by the standard CLEAN method (i.e., super-resolution). A demonstration with a VLBA 7 mm example data set shows consistent results with a near-in-time 3 mm VLBI image. Application of the method to the 2009 GMVA data yields an image with remarkable fine-scale structures that have been never imaged before. We present a brief interpretation of the complexity in the structure.

Jets of compact radio sources are highly relativistic and Doppler boosted, making studies of their intrinsic properties difficult. Observed brightness temperatures can be used to study the intrinsic physical properties of the relativistic jets, and constrain models of jet formation in the inner jet region. We aim to observationally test such inner jet models. The very long baseline interferometry (VLBI) cores of compact radio sources are optically thick at a given frequency. The distance of the core from the central engine is inversely proportional to the frequency. Under the equipartition condition between the magnetic field energy and particle energy densities, the absolute distance of the VLBI core can be predicted. We compiled the brightness temperatures of VLBI cores at various radio frequencies of 2, 8, 15, and 86~GHz. We derive the brightness temperature on sub-parsec scales in the rest frame of the compact radio sources. We find that the brightness temperature increases with increasing distance from the central engine, indicating that the intrinsic jet speed (the Lorentz-factor) increases along the jet. This implies that the jets are accelerated in the (sub-)parsec regions from the central engine., Comment: 14 pages, 3 figures, 2 tables, accepted for publication in ApJ

The detailed investigation of the broadband flux variability in the blazar 3C 273 allowed us to probe the location and size of emission regions and their physical conditions. We conducted correlation studies of the flaring activity in 3C 273, which was observed for the period between 2008 and 2012. The observed broadband variations were investigated using the structure function and the discrete correlation function methods. Starting from the commonly used power spectral density (PSD) analysis at X-ray frequencies, we extended our investigation to characterise the nature of variability at radio, optical, and γ -ray frequencies. The PSD analysis showed that the optical and infrared light-curve slopes are consistent with the slope of white-noise processes, while the PSD slopes at radio, X-ray, and γ -ray energies are consistent with red-noise processes. We found that the estimated fractional variability amplitudes strongly depend on the observed frequency. The flux variations at γ -ray and mm-radio bands are found to be significantly correlated. Using the estimated time lag of (110 ± 27) days between γ -ray and radio light-curves, where γ -ray variations lead the radio bands, we constrained the location of the γ -ray emission region at a de-projected distance of 1.2 ± 0.9 pc from the jet apex. Flux variations at X-ray bands were found to have a significant correlation with variations at both radio and γ -ray energies. The correlation between X-ray and γ -ray light curves indicates two possible time lags, which suggests that two components are responsible for the X-ray emission. A negative time lag of −(50 ± 20) days, where the X-rays are leading the emission, suggests that X-rays are emitted closer to the jet apex from a compact region (0.02–0.05 pc in size), most likely from the corona at a distance of (0.5 ± 0.4) pc from the jet apex. A positive time lag of (110 ± 20) days (γ -rays are leading the emission) suggests a jet-base origin of the other X-ray component at ~4 to 5 pc from the jet apex. The flux variations at radio frequencies were found to be well correlated with each other such that the variations at higher frequencies are leading the lower frequencies, which is expected from the standard shock-in-jet model.

Using high-resolution radio imaging with VLBI techniques, the TANAMI program has been observing the parsec-scale radio jets of southern (declination south of -30{\deg}) gamma-ray bright AGN simultaneously with Fermi/LAT monitoring of their gamma-ray emission. We present the radio and gamma-ray properties of the TANAMI sources based on one year of contemporaneous TANAMI and Fermi/LAT data. A large fraction (72%) of the TANAMI sample can be associated with bright gamma-ray sources for this time range. Association rates differ for different optical classes with all BL Lacs, 76% of quasars and just 17% of galaxies detected by the LAT. Upper limits were established on the gamma-ray flux from TANAMI sources not detected by LAT. This analysis led to the identification of three new Fermi sources whose detection was later confirmed. The gamma-ray and radio luminosities are related by $L_\gamma \propto L_r^{0.89+-0.04}$. The brightness temperatures of the radio cores increase with the average gamma-ray luminosity, and the presence of brightness temperatures above the inverse Compton limit implies strong Doppler boosting in those sources. The undetected sources have lower gamma/radio luminosity ratios and lower contemporaneous brightness temperatures. Unless the Fermi/LAT-undetected blazars are strongly gamma-ray-fainter than the Fermi/LAT-detected ones, their gamma-ray luminosity should not be significantly lower than the upper limits calculated here., Comment: 12 pages, 11 figures, 3 tables, Accepted for publication in Astronomy & Astrophysics

Context. Blazars are among the most energetic objects in the Universe. In 2008 August, Fermi/LAT detected the blazar PKS 1502+106 showing a rapid and strong gamma-ray outburst followed by high and variable flux over the next months. This activity at high energies triggered an intensive multi-wavelength campaign covering also the radio, optical, UV, and X-ray bands indicating that the flare was accompanied by a simultaneous outburst at optical/UV/X-rays and a delayed outburst at radio bands. Aims: In the current work we explore the phenomenology and physical conditions within the ultra-relativistic jet of the gamma-ray blazar PKS 1502+106. Additionally, we address the question of the spatial localization of the MeV/GeV-emitting region of the source. Methods: We utilize ultra-high angular resolution mm-VLBI observations at 43 and 86 GHz complemented by VLBI observations at 15 GHz. We also employ single-dish radio data from the F-GAMMA program at frequencies matching the VLBI monitoring. Results: PKS 1502+106 shows a compact core-jet morphology and fast superluminal motion with apparent speeds in the range 5--22 c. Estimation of Doppler factors along the jet yield values between ~7 up to ~50. This Doppler factor gradient implies an accelerating jet. The viewing angle towards the source differs between the inner and outer jet, with the former at ~3 degrees and the latter at ~1 degree, after the jet bends towards the observer beyond 1 mas. The de-projected opening angle of the ultra-fast, magnetically-dominated jet is found to be (3.8 +/- 0.5) degrees. A single jet component can be associated with the pronounced flare both at high-energies and in radio bands. Finally, the gamma-ray emission region is localized at less than 5.9 pc away from the jet base., Comment: 22 pages, 12 figures. Accepted for publication in A&A

In this Letter, we omitted to cite preliminary results from the low-energy extension of the Pierre Auger Observatory, as presented at the International Cosmic Ray Conference 2015 (ref. 1). Figure 1 of this Corrigendum shows measurements of the average value of Xmax for the Low Frequency Array (LOFAR), and earlier experiments using different techniques, now including the data from the Pierre Auger Observatory1 , specifically the contribution of A. Porcelli. Our values are in agreement with those of ref. 1 within systematic uncertainties.

We report the development of a semi-automatic pipeline for the calibration of 86 GHz full-polarization observations performed with the Global Millimeter-VLBI array (GMVA) and describe the calibration strategy followed in the data reduction. Our calibration pipeline involves non-standard procedures, since VLBI polarimetry at frequencies above 43 GHz is not yet well established. We also present, for the first time, a full-polarization global-VLBI image at 86 GHz (source 3C 345), as an example of the final product of our calibration pipeline, and discuss the effect of instrumental limitations on the fidelity of the polarization images. Our calibration strategy is not exclusive for the GMVA, and could be applied on other VLBI arrays at millimeter wavelengths. The use of this pipeline will allow GMVA observers to get fully-calibrated datasets shortly after the data correlation., 10 pages, 10 figures. Accepted for publication in A&A

We report on the high precision timing analysis of the pulsar-white dwarf binary PSR J1012+5307. Using 15 years of multi-telescope data from the European Pulsar Timing Array (EPTA) network, a significant measurement of the variation of the orbital period is obtained. Using this ideal strong-field gravity laboratory we derive theory independent limits for both the dipole radiation and the variation of the gravitational constant., 3 pages, Proceedings of the 12th Marcel Grossmann Meeting on General Relativity (MG 12)

We have investigated a frequency-dependent shift in the absolute position of the optically thick apparent origin of parsec-scale jets ("core shift" effect) to probe physical conditions in ultra-compact relativistic outflows in AGN. We used multi-frequency Very Long Baseline Array (VLBA) observations of 191 sources carried out in 12 epochs in 2006 within the MOJAVE program. The observations were performed at 8.1, 8.4, 12.1, and 15.4 GHz. We implemented a method of determining the core shift vector based on (i) image registration by two-dimensional normalized cross-correlation and (ii) model-fitting the source brightness distribution to take into account a non-zero core component offset from the phase center. The 15.4-8.1, 15.4-8.4, and 15.4-12.1 GHz core shift vectors are derived for 163 sources, and have median values of 0.128, 0.125, and 0.088 mas, respectively, compared to the typical measured errors of 0.050, 0.051, 0.035 mas. The effect occurs predominantly along the jet direction, with departures smaller than 45 deg from the median jet position angle in over 80% of the cases. Despite the moderate ratio of the observed frequencies (2sigma) are detected for about 55% of the sources. These shifts are even better aligned with the jet direction, deviating from the latter by less than 30 deg in over 90% of the cases. There is an indication that the core shift decreases with increasing redshift. Magnetic fields in the jet at a distance of 1 parsec from the central black hole, calculated from the obtained core shifts, are found to be systematically stronger in quasars (median B1~0.9 G) than those in BL Lacs (median B1~0.4 G). We also constrained the absolute distance of the core from the apex of the jet at 15 GHz as well as the magnetic field strength in the 15 GHz core region., 10 pages, 10 figures, accepted for publication in A&A on 20 July, 2012

The F-GAMMA program is a coordinated effort to investigate the physics of Active Galactic Nuclei (AGNs) via multi-frequency monitoring of Fermi blazars. In the current study we show and discuss the evolution of broad-band radio spectra, which are measured at ten frequencies between 2.64 and 142 GHz using the Effelsberg 100-m and the IRAM 30-m telescopes. It is shown that any of the 78 sources studied can be classified in terms of their variability characteristics in merely 5 types of variability. It is argued that these can be attributed to only two classes of variability mechanisms. The first four types are dominated by spectral evolution and can be described by a simple two-component system composed of: (a) a steep quiescent spectral component from a large scale jet and (b) a time evolving flare component following the "Shock-in-Jet" evolutionary path. The fifth type is characterised by an achromatic change of the broad band spectrum, which could be attributed to a different mechanism, likely involving differential Doppler boosting caused by geometrical effects. Here we present the classification, the assumed physical scenario and the results of calculations that have been performed for the spectral evolution of flares., Comment: Proceedings of the conference: "The Central Kiloparsec in Galactic Nucleic: Astronomy at High Angular Resolution 2011", August 29 - September 2, 2011, Bad Honnef, Germany

The mini-spiral is a feature of the interstellar medium in the central ~2 pc of the Galactic center. It is composed of several streamers of dust and ionised and atomic gas with temperatures between a few 100 K to 10^4 K. There is evidence that these streamers are related to the so-called circumnuclear disk of molecular gas and are ionized by photons from massive, hot stars in the central parsec. We attempt to constrain the emission mechanisms and physical properties of the ionized gas and dust of the mini-spiral region with the help of our multiwavelength data sets. Our observations were carried out at 1.3 mm and 3 mm with the mm interferometric array CARMA in California in March and April 2009, with the MIR instrument VISIR at ESO's VLT in June 2006, and the NIR Br-gamma with VLT NACO in August 2009. We present high resolution maps of the mini-spiral, and obtain a spectral index of 0.5 for Sgr A*, indicating an inverted synchrotron spectrum. We find electron densities within the range 0.8-1.5x10^4 cm-3 for the mini-spiral from the radio continuum maps, along with a dust mass contribution of ~0.25 solar masses from the MIR dust continuum, and extinctions ranging from 1.8-3 at 2.16 micron in the Br-gamma line. We observe a mixture of negative and positive spectral indices in our 1.3 mm and 3 mm observations of the extended emission of the mini-spiral, which we interpret as evidence that there are a range of contributions to the thermal free-free emission by the ionized gas emission and by dust at 1.3 mm., Comment: 9 pages, 11 figures, accepted to A&A