Hadronic Gamma Rays from Supernova Remnants

TL;DR

This paper explores gamma-ray emission from gas clouds near supernova remnants, highlighting how particle leakage affects the gamma-ray spectrum, especially at very-high energies, and applies findings to Galactic center observations.

Contribution

It introduces a model for gamma-ray emission considering particle leakage from SNR shocks, challenging the standard power-law assumption for ambient particle spectra.

Findings

Gamma-ray spectra are flatter than expected when particles leak from SNRs.

The cutoff in gamma-ray spectra is determined by pion momentum distribution.

Application to HESS data explains diffuse gamma-ray emission at the Galactic center.

Abstract

A gas cloud near a supernova remnant (SNR) provides a target for pp-collisions leading to subsequent gamma-ray emission through neutral pion decay. The assumption of a power-law ambient spectrum of accelerated particles with index near -2 is usually built into models predicting the spectra of very-high energy (VHE) gamma-ray emission from SNRs. However, if the gas cloud is located at some distance from the SNR shock, this assumption is not necessarily correct. In this case, the particles which interact with the cloud are those leaking from the shock and their spectrum is approximately monoenergetic with the injection energy gradually decreasing as the SNR ages. In the GLAST energy range the gamma-ray spectrum resulting from particle interactions with the gas cloud will be flatter than expected, with the cutoff defined by the pion momentum distribution in the laboratory frame. We evaluate the flux of particles escaping from a SNR shock and apply the results to the VHE diffuse emission detected by the HESS at the Galactic centre.