Extragalactic background light absorption signal in the 0.26-10 TeV spectra of blazars

Abstract

Recent observations of the TeV gamma-ray spectra of the two closest active galactic nuclei (AGNs), Markarian 501 (Mrk 501) and Markarian 421 (Mrk 421), by the Whipple and HEGRA collaborations have stimulated efforts to estimate or limit the spectral energy density (SED) of extragalactic background light (EBL) which causes attenuation of TeV photons via pair-production when they travel cosmological distances. In spite of the lack of any distinct cutoff-like feature in the spectra of Mrk 501 and Mrk 421 (in the interval 0.26-10 TeV) which could clearly indicate the presence of such a photon absorption mechanism, we demonstrate that strong EBL attenuation signal (survival probability of 10 TeV photon (~10^{-2}) may still be present in the spectra of these AGNs. This attenuation could escape detection due to a special form of SED of EBL and unknown intrinsic spectra of these blazars. Here we show how the proposed and existing experiments, VERITAS, HESS, MAGIC, STACEE and CELESTE may be able to detect or severely limit the EBL SED by extension of spectral measurements into the critical 100-300 GeV regime.