Gamma-rays from Blazars and the Extragalactic Background Light (EBL)

Abstract

Gamma-rays from blazars at high energy (HE, 0.1–100 GeV) and very high energy (VHE, >0.1 TeV) have constrained the intensity and spectrum of the diffuse Extragalactic Background Light (EBL) to be close to the level given by galaxy counts, and in agreement with the most recent models of galaxy evolution. As a consequence, they indicate the starlight from normal galaxies as the predominant origin of the EBL, excluding a strong contribution from other sources like PopIII stars. A low EBL means that future detections at VHE of sources as far as z= 1 or even 1.5 are indeed possible and expected, without invoking new and exotic physics. The combination of HE and VHE spectra has also allowed to put important lower limits on the intergalactic magnetic field (IGMF), from the absence of evidence for the secondary emission expected from the electronpositron pairs produced in gamma-gamma collisions with the EBL photons. However some issues remain open, in particular in the mid and far-infrared bands and in the blazar spectra at multi-TeV energies. This energy range, probed by the new and upcoming facilities like ASTRI, HAWC and CTA, remains the most promising for significant improvements in our understanding of both the EBL and blazar physics.