I. Vovk

Evidence for strong extragalactic magnetic fields from Fermi observations of TeV blazars.

On the Origins of Magnetism The magnetic fields in galaxies and galaxy clusters are thought to result from the amplification of weak primordial magnetic fields, which, according to one class of theories, should exist in the voids between galaxies and galaxy clusters. Neronov and Vovk (p. 73) present evidence for the existence of intergalactic magnetic fields and derive a lower limit for their strength, based on an analysis of data from the Fermi Large Area Telescope. The results place constraints on magnetogenesis models and suggest that magnetic fields originated in the early universe before galaxy formation took place. An analysis of data from the Fermi Large Area Telescope sets a lower limit for the strength of intergalactic magnetic fields. Magnetic fields in galaxies are produced via the amplification of seed magnetic fields of unknown nature. The seed fields, which might exist in their initial form in the intergalactic medium, were never detected. We report a lower bound B ≥ 3 × 10−16 gauss on the strength of intergalactic magnetic fields, which stems from the nonobservation of GeV gamma-ray emission from electromagnetic cascade initiated by tera–electron volt gamma rays in intergalactic medium. The bound improves as λB−1/2 if magnetic field correlation length, λB, is much smaller than a megaparsec. This lower bound constrains models for the origin of cosmic magnetic fields.

Extragalactic magnetic fields constraints from simultaneous GeV-TeV observations of blazars

Attenuation of the TeV gamma-ray flux from distant blazars through pair production with extragalactic background light leads to the development of electromagnetic cascades and subsequent, lower energy, GeV secondary gamma-ray emission. Due to the deflection of VHE cascade electrons by extragalactic magnetic fields (EGMF), the spectral shape of this arriving cascade gamma-ray emission is dependent on the strength of the EGMF. Thus, the spectral shape of the GeV-TeV emission from blazars has the potential to probe the EGMF strength along the line of sight to the object. We investigate constraints on the EGMF derived from observations of blazars for which TeV observations simultaneous with those by the Fermi telescope were reported. We study the dependence of the EGMF bound on the hidden assumptions it rests upon. We select blazar objects for which simultaneous Fermi/LAT GeV and Veritas, MAGIC or HESS TeV emission have been published. We model the development of electromagnetic cascades along the gamma-ray beams from these sources using Monte Carlo simulations, including the calculation of the temporal delay incurred by cascade photons, relative to the light propagation time of direct gamma-rays from the source. Constraints on EGMF could be derived from the simultaneous GeV-TeV data on the blazars RGB J0710+591, 1ES 0229+200, and 1ES 1218+304. The measured source flux level in the GeV band is lower than the expected cascade component calculated under the assumption of zero EGMF. Assuming that the reason for the suppression of the cascade component is the extended nature of the cascade emission, we find that B>10^{-15} G (assuming EGMF correlation length of ~1 Mpc) is consistent with the data. Alternatively, the assumption that the suppression of the cascade emission is caused by the time delay of the cascade photons the data are consistent with B>10^{-17} G for the same correlation length.

FERMI/LAT OBSERVATIONS OF 1ES 0229+200: IMPLICATIONS FOR EXTRAGALACTIC MAGNETIC FIELDS AND BACKGROUND LIGHT

We report the observation in the GeV band of the blazar 1ES 0229+200, which over recent years has become one of the primary sources used to put constraints on the extragalactic background light (EBL) and extragalactic magnetic field (EGMF). We derive constraints on both the EBL and EGMF from the combined Fermi-HESS data set taking into account the direct and cascade components of the source spectrum. We show that the limit on the EBL depends on the EGMF strength and vice versa. In particular, an EBL density twice as high as that derived by Franceschini et al. in 2008 is allowed if the EGMF is strong enough. On the other hand, an EGMF strength as low as 6 × 10–18 G is allowed if the EBL density is at the level of the lower bound from the direct source counts. We present the combined EBL and EGMF limits as an exclusion plot in two-dimensional parameter space: EGMF strength versus EBL density.

Variability of Gamma-Ray Emission from Blazars on Black Hole Timescales

We investigate the variability properties of blazars in the GeV band using data from the Fermi/Large Area Telescope (LAT) telescope. We find that blazars exhibit variability down to the minimum timescale resolvable by Fermi; this variability is a function of the peak photon count rate in the LAT. This implies that the real minimum variability timescales for the majority of blazars are typically shorter than those resolvable by the LAT. We find that for several blazars these minimum variability timescales reach those associated with the blazar central engine, the supermassive black hole. At the same time, none of the blazars exhibits variability on a timescale shorter than the black hole horizon light-crossing time and/or the period of rotation around the last stable circular orbit. Based on this fact, we argue that the timing properties of the γ-ray signal could be determined by the processes in the direct vicinity of the supermassive black hole.

Microlensing constraints on the size of the gamma-ray emission region in blazar B0218+357

Context. Observations of the effect of microlensing in gravitationally lensed quasars could potentially be used to study the structure of the source on distance scales down to the size of the supermassive black hole powering the quasar activity. Aims. We search for the microlensing effect in the gamma-ray band using the signal from a gravitationally lensed blazar B0218+357. Methonds. We develop a method of deconvolution of contributions of two images of the source into the gamma-ray band flaring lightcurve. We use this method to study the evolution of the magnification factor ratio between the two images throughout the flaring episodes. We interpret the time variability of the ratio as a signature of the microlensing effect and derive constraints on the physical parameters of the gamma-ray source by comparing the observed variability properties of the magnification factor ratio with those derived from numerical simulations of the microlensing caustics networks. Results. We find that the magnification factor ratio has experienced a change characteristic for a microlensing caustic crossing event during a 100 d flaring period in 2012. It has further changed between 2012 and a recent flaring episode in 2014. We use the measurement of the maximal magnification and duration of the caustic crossing event to derive an estimate of the projected size of the gamma-ray emission region in B0218+357,

Fast variability of γ‐ray emission from supermassive black hole binary OJ 287

R_\gamma \sim 10^{14}

Minimal variability time scale – central black hole mass relation of the γ-ray loud blazars

cm. This estimate is compatible with a complementary estimate found from the minimal variability time scale. The microlensing / minimal variability time scale measurements of the source size suggest that the gamma-ray emission is produced at the base of the blazar jet, in the direct vicinity of the central supermassive black hole.

MAGIC observations of the February 2014 flare of 1ES 1011+496 and measurement of the Extragalactic Background Light density

We report the discovery of fast variability of γ-ray flares from blazar OJ 287. This blazar is known to be powered by a binary system of supermassive black holes. The observed variability time-scale T var ≲ 3-10 h is much shorter than the light-crossing time of the more massive (1.8 x 10 10 M ⊙ ) black hole and is comparable to the light-crossing time of the less massive (1.3 x 10 8 M ⊙ ) black hole. This indicates that γ-ray emission is produced by the relativistic jet ejected by the black hole of smaller mass. Detection of γ-rays with energies in excess of 10 GeV during the fast variable flares constrains the Doppler factor of the jet to be larger than 4. The possibility of studying the orbital modulation of emission from the relativistic jet makes OJ 287 a unique laboratory for the study of the mechanism(s) of formation of jets by black holes, and in particular the response of the jet parameters to changes in the parameters of the medium from which the black hole accretes and into which the jet expands.

Cosmic-ray composition measurements and cosmic ray background-free

Context. The variability time scales of the blazar γ-ray emission contain the imprints of the sizes of their emission zones and are generally expected to be larger than the light-crossing times of these zones. In several cases the time scales were found to be as short ∼10 min, suggesting that the emission zone sizes are comparable with the sizes of the central supermassive black holes. Previously, these measurements also led to the suggestion of a possible connection between the observed minimal variability time scales and the masses of the corresponding black holes. This connection can be used to determine the location of the γ-ray emission site, which currently remains uncertain. Aims. The study aims to investigate the suggested “minimum time scale – black hole mass” relation using the blazars, detected in the TeV band. Methods. To obtain the tightest constraints on the variability time scales this work uses a compilation of observations by the Cherenkov telescopes HESS, MAGIC, and VERITAS. These measurements are compared to the blazar central black hole masses found in the literature. Results. The majority of the studied blazars show the variability time scales which are at least comparable to the period of rotation along the last stable orbit of the central black hole – and in some cases as short as its light-crossing time. For several sources the observed variability time scales are found to be smaller than the black hole light-crossing time. This suggests that the detected γ-ray variability originates, most probably, from the turbulence in the jet, sufficiently far from the central black hole.

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1ES 1011+496 is a blazar located at a redshift z=0.212, revealed as a very-high-energy γ-ray emitter by MAGIC in 2007. In February 2014 the source underwent an unprecedented flaring episode. Following a flare alert issued by VERITAS, the MAGIC telescopes carried out an observation campaign for a total of 17 nights between February 6 and March 7, during which the source reached a peak flux of almost 14 times the flux measured by MAGIC at the time of discovery, before returning to its low state. Despite the large flux variations, the estimated intrinsic spectral shape was remarkably stable through the whole period. The average spectrum during the flare could be well measured up to a few TeV, which makes it an ideal observation for probing the Extragalactic Background Light (EBL) through its effects on the γ-ray flux. We implemented a method similar to the one used recently in highand very-high-energy γ-ray astronomy for this purpose, consisting in a likelihood maximization in which both the intrinsic spectral parameters of the source and the EBL density are free parameters. With this method we computed limits on the EBL density using as a template Domínguez et al. (2011) model with an additional scaling parameter. This measurement is among the most constraining ones obtained with γ-ray telescopes on a single source and strengthens the case for no significant contribution of unresolved sources to the EBL.