The case for a low extragalactic gamma-ray background

Measurements of the diffuse extragalactic gamma-ray background (EGRB) are complicated by a strong Galactic foreground. Estimates of the EGRB flux and spectrum, obtained by modelling the Galactic emission, have produced a variety of (sometimes conflicting) results. The latest analysis of the EGRET data found an isotropic flux I-x=1.45+/-0.05 above 100 MeV, in units of 10(-5) ph s(-1) cm(-2) sr(-1). We analyse the EGRET data in search of robust constraints on the EGRB flux, finding the gamma-ray sky strongly dominated by Galactic foreground even at high latitudes, with no conclusive evidence for an additional isotropic component. The gamma-ray intensity measured towards the Galactic poles is similar to or lower than previous estimates of I-x, even before Galactic foreground subtraction. The high-latitude profile of the gamma-ray data is disc-like for 40degreesless than or similar to\b\less than or similar to70degrees, and even steeper for \b\greater than or similar to70degrees; overall it exhibits strong Galactic features and is well fit by a simple Galactic model. Based on the \b\>40degrees data we find that I-x<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 I-x, 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 favoured by our analysis places stringent limits on extragalactic scenarios involving gamma-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 gamma-ray missions such as GLAST and AGILE could indirectly identify the EGRB.