Intergalactic absorption and blazar gamma-ray spectra

TL;DR

This paper investigates why TeV blazar spectra show no clear trend with redshift, attributing it to a combination of intergalactic absorption effects and intrinsic emission properties of blazars.

Contribution

It proposes a combined explanation involving intergalactic gamma-ray absorption and intrinsic blazar emission features to account for observed spectral distributions.

Findings

Spectral slopes show no clear redshift trend due to absorption and intrinsic factors.

Flux bias favors detection of flaring, flatter-spectrum sources at higher redshifts.

Intergalactic absorption progressively steepens spectra, obscuring intrinsic correlations.

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 gamma-gamma 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.