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

Abstract

In this work we study the individual contribution to diffuse γ-ray emission from the secondary products in hadronic interactions generated by cosmic rays (CRs), in addition to the contribution of π 0 decay via the decay mode π 0 → 2γ. For that purpose we employ the Monte Carlo particle collision code DPMJET3.04 to determine the multiplicity spectra of various secondary particles with γ’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 γ-ray production by cosmic rays with energies below a few GeV, where DPMJET appears unreliable, we thus derive an easy-to-use γ-ray production matrix for cosmic rays with energies up to about 10 PeV. We apply the γ-ray production matrix to the GeV excess in diffuse galactic γ-rays observed by EGRET. Although the non-π 0 decay components have contributed to the total emission with a different spectrum from the π 0-decay component, they are insufficient to explain the GeV excess. 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 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 χ 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.