Probing cluster environments of blazars through gamma-gamma absorption

Abstract

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 the intracluster medium that produce diffuse intracluster light. These radiation fields may absorb very high energy gamma rays ($E\gtrsim 100$ 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, $\gtrsim 5$, luminous nearby galaxies. This situation is not found in the local Universe, but may be possible at larger redshifts ($z\gtrsim 2$). 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 $\sim 10$ nearby galaxies.

5 pages, 5 figures, accepted for publication in A&A

References in corpus (5)

• Search for TeV Gamma-ray Emission from GRB 100621A, an extremely bright GRB in X-rays, with H.E.S.S
• Discovery of Very High Energy Gamma-Ray Emission from the BL Lac Object H2356-309 with the H.E.S.S. Cherenkov Telescopes
• Optical spectroscopy of BL Lacertae objects. Broad lines, companion galaxies and redshift lower limits
• VHE Gamma-Ray Induced Pair Cascades in the Radiation Fields of Dust Tori of AGN: Application to Cen A
• How gravitational lensing helps gamma-ray photons avoid $\gamma -\gamma$ absorption

Cited by in corpus (1)

• EBL Inhomogeneity and Hard-Spectrum Gamma-Ray Sources