Gamma-Rays Produced in Cosmic-Ray Interactions and the TeV-band Spectrum of RX J1713.7-3946

TL;DR

This paper uses Monte Carlo simulations to model gamma-ray production from cosmic-ray interactions, applying it to Galactic gamma-ray observations and supernova remnant spectra, revealing insights into cosmic-ray acceleration and gamma-ray origins.

Contribution

It provides a new gamma-ray production matrix for cosmic rays up to 10 PeV and applies it to explain gamma-ray observations, testing hadronic models for SNR RX J1713.7-3946.

Findings

Non-$ ext{π}^0$ decay components are insufficient to explain the GeV excess.

A continuously softening spectrum is preferred for the SNR gamma-ray emission.

Hard spectra with cutoff are unlikely to explain the HESS data.

Abstract

We employ the Monte Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles (in addition to $\pi^0$'s) with $\gamma$'s as the final decay state, that are produced in cosmic-ray ($p$'s and $\alpha$'s) interactions with the interstellar medium. We derive an easy-to-use $\gamma$-ray production matrix for cosmic rays with energies up to about 10 PeV. This $\gamma$-ray production matrix is applied to the GeV excess in diffuse Galactic $\gamma$-rays observed by EGRET, and we conclude the non-$\pi^0$ decay components are insufficient to explain the GeV excess, although they have contributed a different spectrum from the $\pi^0$-decay component. We also test the hypothesis that the TeV-band $\gamma$-ray emission of the shell-type SNR RX J1713.7-3946 observed with HESS is caused by hadronic cosmic rays which are accelerated by a cosmic-ray modified shock. By the $\chi^2$ statistics, we find a continuously softening spectrum is strongly preferred, in contrast to expectations. A hardening spectrum has about 1% probability to explain the HESS data, but then only if a hard cutoff at 50-100 TeV is imposed on the particle spectrum.