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.
We investigate potential $gamma-gamma$ absorption of gamma-ray emission from blazars arising from inhomogeneities along the line of sight, beyond the diffuse Extragalactic Background Light (EBL). As plausible sources of excess $gamma-gamma$ opacity,
we consider (1) foreground galaxies, including cases in which this configuration leads to strong gravitational lensing, (2) individual stars within these foreground galaxies, and (3) individual stars within our own galaxy, which may act as lenses for microlensing events. We found that intervening galaxies close to the line-of-sight are unlikely to lead to significant excess $gamma-gamma$ absorption. This opens up the prospect of detecting lensed gamma-ray blazars at energies above 10 GeV with their gamma-ray spectra effectively only affected by the EBL. The most luminous stars located either in intervening galaxy or in our galaxy provides an environment in which these gamma-rays could, in principle, be significantly absorbed. However, despite a large microlensing probability due to stars located in intervening galaxies, gamma-rays avoid absorption by being deflected by the gravitational potentials of such intervening stars to projected distances (impact parameters) where the resulting $gamma-gamma$ opacities are negligible. Thus, neither of the intervening excess photon fields considered here, provide a substantial source of excess $gamma-gamma$ opacity beyond the EBL, even in the case of very close alignments between the background blazar and a foreground star or galaxy.
