Constraints on Extragalactic Background Light from Cherenkov telescopes: status and perspectives for the next 5 years

TL;DR

This paper reviews how very high energy gamma-ray observations from distant blazars constrain the extragalactic background light, discusses current limitations, and explores future prospects with combined gamma-ray observations.

Contribution

It provides an overview of EBL constraints from gamma-ray telescopes, highlighting current limitations and future perspectives for joint Fermi and Cherenkov telescope efforts.

Findings

Recent detections of distant blazars' hard spectra constrain EBL density.

Spectral softening could be intrinsic or due to EBL interaction.

Joint observations can improve EBL constraints.

Abstract

Very high energy (VHE, E > 30 GeV) gamma-rays are absorbed via interaction with low-energy photons from the extragalactic background light (EBL) if the involved photon energies are above the threshold for electron-positron pair creation. The VHE gamma-ray absorption, which is energy dependent and increases strongly with redshift, distorts the VHE energy spectra observed from distant objects. The observed energy spectra of the AGNs carry therefore an imprint of the EBL. Recent detections of hard spectra of distant blazars (z = 0.11 - 0.54) by H.E.S.S. and MAGIC put strong constraints on the EBL density in the optical to near infrared waveband. It is, however, not yet possible to distinguish between an intrinsic softening of blazar spectra and a softening caused by the interaction with low energy EBL photons. In this paper, we give an overview of the EBL constraints, their limitations and perspectives for the joint efforts of the Fermi Gamma-Ray Space telescope and imaging atmospheric Cherenkov telescopes.