Effects of Gamma-Ray Spectral Variability on Fundamental Physics Measurements

Abstract

The Fermi-LAT telescope is used to study the effects of spectral variability upon physical measurements made using gamma-ray data. Firstly, two individual AGN are studied, namely Centaurus A and IC 310. In both cases, the spectrum shows a significant hardening above GeV energies. In the case of Cen A, this is shown to be an intrinsic effect, with little variability to be found. On the other hand, in IC 310, this hardening is shown to be an artefact of spectral variability. To assess the prevalence of spectral variability in the population of AGN, first the statistical tools to assign confidence to our measurements are developed. 302 bright BL Lac objects and FSRQs are then analysed to characterise the variability of the AGN population. Over half of the AGN studied were shown to be spectrally variable at the 5σ level, with a far higher fraction variable in flux. This information is then applied to measurements of the extragalactic background light attenuation found in these gamma-ray AGN, revealing a bias towards higher EBL intensity caused by spectrally variable AGN in the sample. We conclude that spectral variability is an effect that must be taken into account in all measurements of fundamental physical parameters from AGN measurements.