Gamma-rays produced in cosmic-ray interactions and the TeV-band spectrum of RX J1713.7-3946

In this work we study the individual contribution to diffuse gamma-ray emission from the secondary products in hadronic interactions generated by cosmic rays (CRs), in addition to the contribution of pi(0) decay via the decay mode pi(0)-> 2 gamma. For that purpose we employ the Monte Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles with gamma's as the final decay state, that result from inelastic collisions between cosmic-ray protons and helium nuclei and the interstellar medium with standard composition. By combining the simulation results with a parametric model of gamma-ray production by cosmic rays with energies below a few GeV, where DPMJET appears unreliable, we thus derive an easy-to-use gamma-ray production matrix for cosmic rays with energies up to about 10 PeV. We apply the gamma-ray production matrix to the GeV excess in diffuse galactic gamma-rays observed by EGRET. Although the non-no decay components have contributed to the total emission with a different spectrum from the no-decay component, they are insufficient to explain the GeV excess. 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 shock-accelerated hadronic cosmic rays. This scenario implies a very high efficacy of particle acceleration, so the particle spectrum is expected to continuously harden toward high energies on account of cosmic-ray modification of the shock. Using the chi(2) statistics we find that 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 cut-off at 50-100 TeV is imposed on the particle spectrum. (c) 2007 Elsevier B.V. All rights reserved.