Reassessing the contribution of long-GRB high-energy afterglows to the isotropic gamma-ray background after GRB 221009A

Abstract

Very-high-energy photons from gamma-ray bursts (GRBs) are absorbed by the extragalactic background light and partly reprocessed into the GeV band. We update previous estimates of the long-GRB contribution to the Fermi-LAT isotropic gamma-ray background (IGRB), using a time-integrated high-energy afterglow template of GRB 221009A, anchored by GeV-TeV data, together with an explicitly specified scaling with prompt energy and beaming. Despite the observed multi-TeV afterglow, the cascade component reaches only 1.6e-4 of the IGRB. Including direct GeV emission raises the maximum fraction to 1.1e-3, but makes the result more dependent on the population-averaged broadband template. Variations of the extragalactic background light, propagation implementation, intrinsic spectral cutoff, and population prescription do not change the conclusion: the multi-TeV detection of GRB 221009A does not make long GRBs a significant source of the IGRB.