Selection of AGN to study the extragalactic background light with HAWC

TL;DR

This paper explores how gamma-ray observations of active galactic nuclei with HAWC can help constrain the extragalactic background light, especially in the 1-100 μm range, by studying gamma-ray opacity.

Contribution

It introduces a method to use HAWC gamma-ray data to measure EBL opacity in a poorly explored wavelength range, complementing existing IACT observations.

Findings

HAWC can detect about 10 AGN in its first year.

Expected to constrain EBL intensity in the 1-100 μm range.

Complementary to existing gamma-ray observations.

Abstract

The extragalactic background light (EBL) is all the electromagnetic energy released by resolved and unresolved extragalactic sources since the recombination era. Its intensity and spectral shape provide information about the evolution of galaxies throughout cosmic history. Since direct observations of the EBL are very difficult to perform, the study of the interaction between the low energy EBL photons and high energy photons from distant extragalactic sources becomes relevant to constrain the EBL intensity. The main goal of this study is to investigate the opacity of the EBL to gamma rays by observing a sample of active galaxies nuclei (AGN) with the High Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory. Current gamma-ray observations up to 20 TeV performed by Imaging Atmospheric Cherenkov Telescopes (IACTs) have constrained the EBL intensity in the 0.1-50 $\mu$m region. HAWC, which monitors the gamma-ray sky in the 100 GeV to 100 TeV energy range, will be able to detect about 10 AGN with the first year of HAWC data based on the extrapolation of steady-state spectra from the GeV band to TeV, and thus constrain the EBL in the poorly-measured 1-100 $\mu$m range.