Impacts of Voids, Line of Sight Interactions, and Local Emission Environment on Detectability of Gamma-Ray AGN

Abstract

Cosmic voids may have novel affects on the propagation of high-energy photons. We consider the fraction of the line of sight that intersect voids (termed \enquote{voidiness}). A previous study showed that active galactic nuclei (AGN) detected by \textit{Fermi} Large Area Telescope (LAT) lie along voidier lines of sight than redshift-matched populations of Sloan Digital Sky Survey (SDSS) optically detected quasars in the redshift range from $0.4\le z<0.7$. We explore this difference and various astrophysical explanations for it. Weaker intergalactic magnetic fields in voids would naturally enhance the gamma-ray cascading flux within the \textit{Fermi}-LAT point-spread function. We find that line-of-sight interactions increasing the flux in the \textit{Fermi}-LAT energy band by $\sim$0.1\% per Mpc of void traversed may be sufficient to result in the observed difference in voidiness distributions. Voidiness comparisons between SDSS QSO and AGN detected by imaging atmospheric Cherenkov telescopes at very-high-energies (VHE) do not yield any conclusive statement, likely because of the limited VHE sample size, and therefore are inconclusive about the role of possibly weaker extragalactic background light within voids. Finally, we measure that $28\pm 3\%$ of gamma-ray detected sources exist within a void (consistent with random mock populations) compared to $19.1\pm 0.3\%$ of SDSS quasars. We do not find any significant local void effect for gamma-ray sources that would explain the voidiness difference between \textit{Fermi}-LAT gamma-ray and SDSS QSO sources. These results suggest that the observed difference in voidiness distributions may be due to line-of-sight interactions rather than the local emission environment of gamma-ray AGN.

12 pages, 6 figures. Accepted for publication in ApJ

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