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

Abstract

We employ the Monte Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles (in addition to $π^0$'s) with $γ$'s as the final decay state, that are produced in cosmic-ray ($p$'s and $α$'s) interactions with the interstellar medium. We derive an easy-to-use $γ$-ray production matrix for cosmic rays with energies up to about 10 PeV. This $γ$-ray production matrix is applied to the GeV excess in diffuse Galactic $γ$-rays observed by EGRET, and we conclude the non-$π^0$ decay components are insufficient to explain the GeV excess, although they have contributed a different spectrum from the $π^0$-decay component. We also test the hypothesis that the TeV-band $γ$-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 $χ^2$ 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.