Diffuse {\gamma}-ray emission from self-confined cosmic rays around Galactic sources

TL;DR

This paper investigates how self-confined cosmic rays around supernova remnants contribute to the diffuse gamma-ray background, emphasizing the role of neutral hydrogen in enhancing secondary particle production.

Contribution

It introduces a model for gamma-ray emission from self-confined cosmic ray halos considering neutral hydrogen effects, linking local cosmic ray confinement to diffuse gamma-ray observations.

Findings

Self-confined cosmic rays can significantly contribute to the diffuse gamma-ray background.

The contribution depends on the neutral hydrogen density around sources.

Halos may explain a substantial part of Fermi-LAT diffuse emission, especially with low neutral density.

Abstract

The propagation of particles accelerated at supernova remnant shocks and escaping the parent remnants is likely to proceed in a strongly non-linear regime, due to the efficient self-generation of Alfv\'en waves excited through streaming instability near the sources. Depending on the amount of neutral hydrogen present in the regions around the sites of supernova explosions, cosmic rays may accumulate an appreciable grammage in the same regions and get self-confined for non-negligible times, which in turn results in an enhanced rate of production of secondaries. Here we calculate the contribution to the diffuse gamma-ray background due to the overlap along lines of sight of several of these extended halos as due to pion production induced by self-confined cosmic rays. We find that if the density of neutrals is low, the halos can account for a substantial fraction of the diffuse emission observed by Fermi-LAT, depending on the orientation of the line of sight with respect to the direction of the Galactic centre.