Research Note

Probing cluster environments of blazars through γγ absorption

¹ Centre for Space Research, North-West University, 2520 Potchefstroom, South Africa
e-mail: iurii.sushch@nwu.ac.za
² Astronomical Observatory of Ivan Franko National University of L’viv, vul. Kyryla i Methodia, 8, 79005 L’viv, Ukraine
³ Astrophysical Institute, Department of Physics and Astronomy, Ohio University, Athens, OH 45701, USA

Received: 24 September 2014
Accepted: 12 October 2014

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 γ rays (VHE; E ≳ 100 GeV) and trigger pair cascades with further production of subsequent generations of γ 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 γγ absorption. Substantial γγ absorption is possible only in the case of multiple, ≳5, luminous nearby galaxies. This situation is not found in the local Universe, but may be possible at larger redshifts (z ≳ 2). Since VHE γ rays from such distances are expected to be strongly absorbed by the extragalactic background light, we consider possible signatures of γ-ray induced pair cascades by calculating the expected GeV flux which appears to be below the Fermi-LAT sensitivity even for ~10 nearby galaxies.