Most blazars are known to be hosted in giant elliptic galaxies, but their cluster environments have not been thoroughly investigated. Cluster environments may contain radiation fields of low-energy photons created by nearby galaxies and/or stars in t
he intracluster medium that produce diffuse intracluster light. These radiation fields may absorb very high energy gamma rays ($Egtrsim100$ GeV; VHE) and trigger pair cascades with further production of subsequent generations of gamma rays with lower energies via inverse Compton scattering on surrounding radiation fields leaving a characteristic imprint in the observed spectral shape. The change of the spectral shape of the blazar reflects the properties of its ambient medium. We show, however, that neither intracluster light nor the radiation field of an individual nearby galaxy can cause substantial gamma-gamma absorption. Substantial gamma-gamma absorption is possible only in the case of multiple, $gtrsim5$, luminous nearby galaxies. This situation is not found in the local Universe, but may be possible at larger redshifts ($zgtrsim2$). Since VHE gamma rays from such distances are expected to be strongly absorbed by the extragalactic background light, we consider possible signatures of gamma-ray induced pair cascades by calculating the expected GeV flux which appears to be below the Fermi sensitivity even for $sim10$ nearby galaxies.
PSR B1259-63/LS 2883 is a very high energy (VHE; $E > 100$ GeV) {gamma}-ray emitting binary consisting of a 48 ms pulsar orbiting around a Be star with a period of $sim3.4$ years. The Be star features a circumstellar disk which is inclined with respe
ct to the orbit in such a way that the pulsar crosses it twice every orbit. The circumstellar disk provides an additional field of target photons which may contribute to inverse Compton scattering and {gamma}{gamma}-absorption, leaving a characteristic imprint in the observed spectrum of the high energy emission. At GeV energies, the source was detected for the first time during the previous periastron passage which took place on December 15, 2010. The Fermi Large Area Telescope (LAT) reported a spectacular and unexpected {gamma}-ray flare occurring around 30 days after periastron and lasting for about 7 weeks. In this paper, we study the signatures of Compton-supported, VHE {gamma}-ray induced pair cascades in the circumstellar disc of the Be star and their possible contribution to the GeV flux. We show that cascade emission generated in the disk cannot be responsible for the GeV flare, but it might explain the GeV emission observed close to periastron. We also show that the {gamma}{gamma}-absorption in the disk might explain the observed TeV light curve.
Supernova remnants (SNRs) are widely considered to be sites of Galactic cosmic ray (CR) acceleration. Vela is one of the nearest Galactic composite SNRs to Earth accompanied by the Vela pulsar and its pulsar wind nebula (PWN) Vela X. The Vela SNR is
one of the most studied remnants and it benefits from precise estimates of various physical parameters such as distance and age. Therefore, it is a perfect object for a detailed study of physical processes in SNRs. The Vela SNR expands into the highly inhomogeneous cloudy interstellar medium (ISM) and its dynamics is determined by the heating and evaporation of ISM clouds. It features an asymmetrical X-ray morphology which is explained by the expansion into two media with different densities. This could occur if the progenitor of the Vela SNR exploded close to the edge of the stellar wind bubble of the nearby Wolf-Rayet star $gamma^2$Velorum and hence one part of the remnant expands into the bubble. The interaction of the ejecta and the main shock of the remnant with ISM clouds causes formation of secondary shocks at which additional particle acceleration takes place. This may lead to the close to uniform distribution of relativistic particles inside the remnant. We calculate the synchrotron radio emission within the framework of the new hydrodynamical model which assumes the supernova explosion at the edge of the stellar wind bubble. The simulated radio emission agrees well with both the total radio flux from the remnant and the complicated radio morphology of the source.
PSR B1259-63/LS 2883 is a binary system consisting of a 48 ms pulsar orbitting around a Be star with an orbital period of ~3.4 years. The system was detected at very high energies (VHE; E > 100 GeV) by the High Energy Stereoscopic System (H.E.S.S.) d
uring its periastron passages in 2004 and 2007. Here we present new H.E.S.S. observations corresponding to its last periastron passage, which occurred on December 15th 2010. These new observations partially overlap with the beginning of a spectacular gamma-ray flare reported by the Fermi-LAT. The H.E.S.S. observations show both flux and spectral properties similar to those reported in previous periastron passages, without any signature of the emission enhancement seen at GeV energies. A careful statistical study based on the Fermi and H.E.S.S. lightcurves leads to the conclusion that the GeV and TeV emission during the flare have a different physical origin. This conlusion, in turn, allows to use Fermi-LAT measurements of the GeV flux as upper limits for the modeling of the VHE emission.
Aim. In this paper we present very high energy (VHE; E>100 GeV) data from the gamma-ray binary system PSR B1259-63/LS 2883 taken around its periastron passage (15th of December 2010) with the High Energy Stereoscopic System (H.E.S.S.) of Cherenkov Te
lescopes. We aim to search for a possible TeV counterpart of the GeV flare detected by the Fermi LAT. In addition, we aim to study the current periastron passage in the context of previous observations taken at similar orbital phases, testing the repetitive behavior of the source. Methods. Observations at VHE were conducted with H.E.S.S. from 9th to 16th of January 2011. The total dataset amounts to around 6 h of observing time. Results. The source is detected in the 2011 data at a significance level of 11.5sigma revealing an averaged integral flux above 1 TeV of (1.01 pm 0.18_{stat} pm 0.20_{sys}) times 10^{-12} cm^{-2}s^{-1}. The differential energy spectrum follows a power-law shape with a spectral index Gamma = 2.92 pm 0.30_{stat} pm 0.20_{sys} and a flux normalisation at 1 TeV of N_{0} = 1.95 pm 0.32_{stat} pm 0.39_{sys}) times 10^{-12} TeV^{-1} cm^{-2} s^{-1}. The measured lightcurve does not show any evidence for variability of the source on the daily scale. Conclusions. The measured integral flux and the spectral shape of the 2011 data are compatible with the results obtained around previous periastron passages. The absence of variability in the H.E.S.S. data indicates that the GeV flare observed by Fermi LAT in the time period covered also by H.E.S.S. observations originates in a different physical scenario than the TeV emission. Additionaly, new results compared to those obtained in the observations which were performed in 2004 at a similar orbital phase, further support the hypothesis of the repetitive behavior of the source.
Supernova remnants (SNRs) are widely considered to be accelerators of cosmic rays (CR). They are also expected to produce very-high-energy (VHE; $E > 100$ GeV) gamma rays through interactions of high-energy particles with the surrounding medium and p
hoton fields. They are, therefore, promising targets for observations with ground-based imaging atmospheric Cherenkov telescopes like the H.E.S.S. telescope array. VHE gamma-ray emission has already been discovered from a number of SNRs, establishing them as a prominent source class in the VHE domain. Of particular interest are the handful of SNRs whose X-ray spectra are dominated by non-thermal synchrotron emission, such as the VHE gamma-ray emitters RX J0852.0-4622 (Vela Jr.) and RX J1713-3946. The shell-type SNRs G1.9+0.3 and G330.2+1.0 also belong to this subclass and are further notable for their young ages ($leq 1$ kyr), especially G1.9+0.3, which was recently determined to be the youngest SNR in the Galaxy ($sim100$ yr). These unique characteristics motivated investigations with H.E.S.S. to search for VHE gamma rays. The results of the H.E.S.S. observations and analyses are presented, along with implications for potential particle acceleration scenarios.
