Fermi Large Area Telescope Constraints on the Gamma-ray Opacity of the Universe

TL;DR

This study uses Fermi LAT observations of distant gamma-ray sources to constrain the universe's gamma-ray opacity, ruling out certain EBL models with high confidence based on observed flux attenuation.

Contribution

It provides new upper limits on gamma-ray opacity across various energies and redshifts, testing and constraining EBL models with Fermi data.

Findings

High-confidence exclusion of the Stecker et al. (2006) baseline EBL model.

Constraints on gamma-ray attenuation up to redshift z~4.3.

Detection of gamma-ray flux consistent with lower EBL intensities.

Abstract

The Extragalactic Background Light (EBL) includes photons with wavelengths from ultraviolet to infrared, which are effective at attenuating gamma rays with energy above ~10 GeV during propagation from sources at cosmological distances. This results in a redshift- and energy-dependent attenuation of the gamma-ray flux of extragalactic sources such as blazars and Gamma-Ray Bursts (GRBs). The Large Area Telescope onboard Fermi detects a sample of gamma-ray blazars with redshift up to z~3, and GRBs with redshift up to z~4.3. Using photons above 10 GeV collected by Fermi over more than one year of observations for these sources, we investigate the effect of gamma-ray flux attenuation by the EBL. We place upper limits on the gamma-ray opacity of the Universe at various energies and redshifts, and compare this with predictions from well-known EBL models. We find that an EBL intensity in the optical-ultraviolet wavelengths as great as predicted by the "baseline" model of Stecker et al. (2006) can be ruled out with high confidence.