The Case for a Low Extragalactic Gamma-ray Background

Measurements of the diffuse extragalactic γ-ray background (EGRB) are complicated by a strong Galactic foreground. Estimates of the EGRB flux and spectrum, obtained by modeling the Galactic emission, have produced a variety of (sometimes conflicting) results. The latest analysis of the EGRET data found an isotropic flux Ix = 1.45 ± 0.05 above 100 MeV, in units of 10−5 ph s−1 cm−2 sr−1. We analyze the EGRET data in search for robust constraints on the EGRB flux, finding the γ-ray sky strongly dominated by Galactic foreground even at high latitudes, with no conclusive evidence for an additional isotropic component. The γ-ray intensity measured towards the Galactic poles is lower than previous estimates of Ix. The high latitude profile of the γ-ray data is disk-like for 40◦ . |b| . 70◦, and even steeper for |b| & 70◦; overall it exhibits strong Galactic features and is well fit by a simple Galactic model. We find that Ix < 0.5 at a 99% confidence level, with evidence for a much lower flux. We show that correlations with Galactic tracers, previously used to identify the Galactic foreground and estimate Ix, are not satisfactory; the results depend on the tracers used and on the part of the sky examined, because the Galactic emission is not linear in the Galactic tracers, and exhibits spectral variations across the sky. The low EGRB flux favored by our analysis places stringent limits on extragalactic scenarios involving γ-ray emission, such as radiation from blazars, intergalactic shocks and production of ultra-high energy cosmic rays and neutrinos. We suggest methods by which future γ-ray missions such as GLAST and AGILE could indirectly identify the EGRB. Subject headings: gamma rays: observations — diffuse radiation — Galaxy: disk — cosmic rays — radiation mechanisms: non-thermal — methods: data analysis