A method for setting upper limits to the extragalactic background light with Fermi-LAT and TeV observations of blazars

TL;DR

This paper introduces a new method combining Fermi-LAT and TeV observations of blazars to set upper limits on the extragalactic background light, constraining EBL models by comparing extrapolated spectra with observed TeV data.

Contribution

The paper presents a novel approach that uses simultaneous gamma-ray observations to derive upper limits on EBL density, improving constraints on cosmic background light models.

Findings

High EBL density models are disfavored by the data.

The method provides firm upper limits on the EBL based on spectral extrapolation.

Simultaneous observations enhance the ability to constrain EBL models.

Abstract

We propose a method for setting upper limits to the extragalactic background light (EBL). Our method uses simultaneous {\em Fermi}-LAT and ground-based TeV observations of blazars and is based on the assumption that the intrinsic spectral energy distribution (SED) of TeV blazars lies below the extrapolation of the {\em Fermi}-LAT SED from GeV to TeV energies. By extrapolating the {\em Fermi}-LAT spectrum, which for TeV blazars is practically unattenuated by photon-photon pair production with EBL photons, a firm upper limit on the intrinsic SED at TeV energies is provided. The ratio of the extrapolated spectrum to the observed TeV spectrum provides upper limits to the optical depth for the propagation of the TeV photons due to pair production on the EBL, which in turn sets firm upper limits to EBL models. We demonstrate our method using simultaneous observations from {\em Fermi}-LAT and ground-based TeV telescopes of the blazars \object{PKS 2155-304} and \object{1ES 1218+304}, and show that high EBL density models are disfavored. We also discuss how our method can be optimized and how {\em Fermi} and X-ray monitoring observations of TeV blazars can guide future TeV campaigns, leading to potentially much stronger constraints on EBL models.