Context. Observations of the effect of microlensing in gravitationally lensed quasars can be used to study the structure of active galactic nuclei on distance scales down to the sizes of a ...supermassive black hole’s powering source activity. Aims. We searched for a microlensing effect in the signal from a gravitationally lensed blazar, B0218+357, in a very-high-energy γ -ray band. Methods. We combined observations of a bright flare of the source in 2014 by the Fermi Large Area Telescope and MAGIC telescopes in the 0.1 − 300 GeV and 65 − 175 GeV energy ranges, respectively. Using the time-delayed leading and trailing signals from two gravitationally lensed images of the source, we measured the magnification factor at the moment of the flare. We used the scaling of the maximal magnification factor with the source size to constrain the size of the γ -ray emission region in the wide 0.1 − 175 GeV energy range. Results. The magnification factor in the very-high-energy band that we derived from our comparison of Fermi /LAT and MAGIC data is μ VHE = 25 −17 +38 , which is substantially larger than the factor found in the radio band. This suggests one of the source images is strongly affected by microlensing at the moment of the flare. Assuming that the microlensing is produced by a stellar mass object in the lens galaxy, we constrained the size of the emission region in the E > 100 GeV band to be R VHE = 6.6 −5.6 +69 × 10 14 cm. We note that the spectrum of the microlensed source was unusually hard at the moment of the flare, and we speculate that this hardening may be due to the energy-dependent microlensing effect. This interpretation suggests that the source size decreases with energy in the entire 0.1 − 175 GeV energy range we studied.
The Tibet AS
γ
collaboration has recently reported the detection of
γ
-rays with energies up to Peta-electronvolt from parts of the Galactic plane. We note that the analysis of
γ
-ray flux by the ...Tibet-AS
γ
experiment also implies an upper bound on the diffuse
γ
-ray flux from high Galactic latitudes (|
b
|> 20°) in the energy range between 100 TeV and 1 PeV. This bound is up to an order of magnitude stronger than previously derived bounds from GRAPES3, KASCADE, and CASA-MIA experiments. We discuss the new Tibet-AS
γ
limit on the high Galactic latitude
γ
-ray flux in the context of possible mechanisms of multi-messenger (
γ
-ray and neutrino) emission from nearby cosmic ray sources, dark matter decays, and the large-scale cosmic ray halo of the Milky Way.
Aims. Despite recent progress in the field, there are still many open questions regarding γ-ray binaries. In this paper we provide an overview of non-transient γ-ray binaries and discuss how ...observations with the Cherenkov Telescope Array (CTA) will contribute to their study. Methods. We simulated the spectral behaviour of the non-transient γ-ray binaries using archival observations as a reference. With this we tested the CTA capability to measure the spectral parameters of the sources and detect variability on various timescales. Results. We review the known properties of γ-ray binaries and the theoretical models that have been used to describe their spectral and timing characteristics. We show that the CTA is capable of studying these sources on timescales comparable to their characteristic variability timescales. For most of the binaries, the unprecedented sensitivity of the CTA will allow studying the spectral evolution on a timescale as short as 30 min. This will enable a direct comparison of the TeV and lower energy (radio to GeV) properties of these sources from simultaneous observations. We also review the source-specific questions that can be addressed with these high-accuracy CTA measurements.
We report on the results of the extensive multi-wavelength campaign from optical to GeV γ-rays of the 2014 periastron passage of PSR B1259−63, which is a unique high-mass γ-ray emitting binary system ...with a young pulsar companion. Observations demonstrate the stable nature of the post-periastron GeV flare and prove the coincidence of the flare with the start of rapid decay of the Hα equivalent width, usually interpreted as a disruption of the Be stellar disc. Intensive X-ray observations reveal changes in the X-ray spectral behaviour happening at the moment of the GeV flare. We demonstrate that these changes can be naturally explained as a result of synchrotron cooling of monoenergetic relativistic electrons injected into the system during the GeV flare.
Delayed ‘pair-echo’ signal from interactions of very-high-energy γ rays in the intergalactic medium can be used for the detection of the intergalactic magnetic field (IGMF). We used the data of the ...Fermi /LAT telescope coupled with LHAASO observatory measurements to confirm the presence of IGMF along the line of sight to the γ -ray burst GRB 221009A. Comparing the Fermi /LAT measurements with the expected level of the pair-echo flux, set by the multi-TeV LHAASO detection, we derived a lower bound 10 −19 G on the IGMF with correlation length l larger than 1 Mpc, improving as l −1/2 for shorter correlation lengths. This provides an independent verification of the existence of a lower bound on IGMF in the voids of the large-scale structure, previously derived from the observations of active galactic nuclei.
SNRs are commonly believed to be the accelerators of the galactic cosmic rays – mainly protons – and are expected to produce γ-rays through the inelastic proton-proton collisions. Fermi/LAT was ...expected to detect many of those, but only a dozen is listed in the recent Fermi/LAT 2nd Source catalogue. To test whether the observed number of SNRs is in agreement with the above assumption, we use a simplified model of an SNR and calculate the predicted amount of the observable remnants taking into account their distribution in the Galaxy and the sensitivity of Fermi/LAT. We find that the observed number of SNRs agrees with the prediction of our model if we assume a low, ≪ 1 cm−3, number density of the SNR's ambient medium. The result, presented here, suggests, that on average the supernova explosions happen in the under-dense regions, such as bubbles, creating by the winds of the progenitor stars. Under this natural supposition our result finds an agreement with the assumption, that the observed population of supernovae remnants is indeed responsible for the production of the galactic cosmic rays.
Context. Observations of the effect of microlensing in gravitationally lensed quasars could potentially be used to study the structure of the source on very small distance scales – down to the size ...of the supermassive black hole, that powers the quasar activity. Aims. We search for the microlensing effect in the γ-ray band using the signal from a gravitationally lensed blazar, B0218+357. Methods. We develop a method to deconvolve the contributions from the two images of the source, which allows us to reconstruct the flaring light curve in the γ-ray band. 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 γ-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 experienced a change characteristic for a caustic-crossing microlensing event that occurred during a 100 d flaring period in 2012. It changed again 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 γ-ray emission region in B0218+357, Rγ ~ 1014−1015 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 γ-ray emission is produced at the base of the blazar jet, in the direct vicinity of the central supermassive black hole.
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 x 10 super(?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.
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 gamma -ray signal could be determined by the processes in the direct vicinity of the supermassive black hole.
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