Impact of the Fluctuations in the Extragalactic Background Light on the $\gamma$-ray Attenuation of the Quasars

Abstract

This work investigates the impact of the extragalactic background light fluctuations on very high energy $\gamma$-ray spectra from distant Quasars. We calculate the extragalactic background light spectral energy distribution using a model that extends those proposed by Razzaque et al. (2009ApJ.697.483R) and Finke et al. (2010ApJ.712.238F). We introduce a model for fluctuations in the extragalactic background light based on fluctuations in the star formation rate density, since these two fluctuations can reasonably be expected to be correlated. Fluctuations in the star formation rate are estimated from the semi-analytical galaxy catalogue of Guo et al. (2013MNRAS.428.1351G), we use his model to derive the resulting opacities for $\gamma$-rays from distant sources. We determine the mean, lower and upper limits for the scatter of the star formation rate density, which then allow us to compute corresponding limits on the extragalactic background light spectrum. We then calculate the impact of these fluctuations limits on the $\gamma$-ray optical depth. The model predicts variations of up to $10\%$ between upper and lower limits for the $\gamma$-ray opacity in the energy range less than $100$ GeV for nearby sources. The impact is found to be smaller but still significant ($\lesssim 5\%$) for very high energy $\gamma$-rays from distant sources.