Intergalactic absorption and blazar γ-ray spectra

¹ INAF and INFN, via G.B.Tiepolo 11, 34143 Trieste, Italy e-mail: persic@oats.inaf.it
² Università di Udine and INFN, via delle Scienze 208, 33100 Udine, Italy
³ Instituto Superior Técnico, Lisboa, Portugal

Received: 15 November 2007
Accepted: 29 January 2008

Abstract

The distribution of TeV spectral slopes versus redshift for currently known TeV blazars (16 sources with z ≤ 0.21, and one with z > 0.25) is essentially a scatter plot with hardly any hint of a global trend. We suggest that this is the outcome of two combined effects of intergalactic  absorption, plus an inherent feature of the SSC (synchro-self-Compton) process of blazar emission. First, flux dimming introduces a bias that favors detection of progressively more flaring sources at higher redshifts. According to mainstream SSC models, more flaring source states imply sources with flatter TeV slopes. This results in a structured relation between intrinsic TeV slope and redshift. The second effect, spectral steepening by intergalactic absorption, affects sources progressively with distance and effectively wipes out the intrinsic slope-redshift correlation.