A new determination of the extragalactic diffuse gamma-ray background from EGRET data

TL;DR

This study uses an optimized GALPROP-based model of Galactic diffuse gamma-ray emission to re-evaluate the extragalactic gamma-ray background (EGRB) using EGRET data from $30\,\,\mathrm{MeV}$ to $50\,\,\mathrm{GeV}$. By fitting high-latitude gamma-ray intensities with a scaled sum of Galactic components and treating the EGRB as the intercept, the authors obtain a spectrum that is lower and exhibits positive curvature relative to a simple power law, as would be expected from blazar contributions. The work highlights significant model-dependent systematics but demonstrates robustness by cross-checking across hemispheres and sky regions. The resulting EGRB spectrum provides tighter constraints on extragalactic gamma-ray sources and supports a blazar-dominated origin with a non-power-law shape. Overall, the paper advances the precision of EGRB estimates and emphasizes the importance of accurate Galactic foreground modeling in high-energy astrophysics.

Abstract

We use the GALPROP model for cosmic-ray propagation to obtain a new estimate of the Galactic component of gamma rays, and show that away from the Galactic plane it gives an accurate prediction of the observed EGRET intensities in the energy range 30 MeV - 50 GeV. On this basis we re-evaluate the extragalactic gamma-ray background. We find that for some energies previous work underestimated the Galactic contribution at high latitudes and hence overestimated the background. Our new background spectrum shows a positive curvature similar to that expected for models of the extragalactic emission based on the blazar population.

Figures (4)

• Figure 1: Observed versus predicted intensities for region $360^\circ<l<0^\circ,10^\circ<|b|<80^\circ$ (GALPROP model ID 500190). Predicted is Galactic only, intercept is EGRB.
• Figure 2: Comparison of our EGRB spectrum (solid, red) as given in Table \ref{['EGRB_flux']} with that from sreekumar98 (dots, magenta). The dashed (blue) line is the sreekumar98 fit: $2.743\times10^{-3} E^{-2.1}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$ MeV$^{-1}$.
• Figure 3: Extragalactic X-ray and $\gamma$-ray spectrum. Data compilation from sreekumar98 except for COMPTEL weidenspointner00 and EGRET 30 MeV -- 20 GeV (this work).
• Figure 4: EGRB spectrum determined for each of the regions in Table \ref{['isotropy']}. Red bars: hemispheres, blue bars: quarter spheres. The upper and lower bound spectra are shown in green, and the adopted EGRB in black. Energies are shifted slightly for clarity. The dashed (blue) line is the sreekumar98 fit.