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

TL;DR

This paper models gamma-ray production from cosmic-ray interactions, assesses their contribution to diffuse Galactic gamma-ray excess, and analyzes the TeV emission from SNR RX J1713.7-3946, finding a softening spectrum is favored.

Contribution

It provides a new gamma-ray production matrix for cosmic rays up to 10 PeV and applies it to explain diffuse gamma-ray excess and SNR observations.

Findings

Non-$$ decay components are insufficient to explain the GeV excess.

A softening cosmic-ray spectrum is strongly preferred for SNR RX J1713.7-3946.

Hard spectra require a cutoff at 50-100 TeV to fit the 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.