Diffuse Gamma-Rays Produced in Cosmic-Ray Interactions and the TeV-band Spectrum of RX J 1713

We employ the Monte Carlo particle collision code DPMJET3.0 4 to determine the multiplicity spectra of various secondary particles (in addition to π’s) with γ’s as the final decay state, that are produced in cosmic-ray ( p’s andα’s) interactions with the interstellar medium. We derive an easy-touseγ-ray production matrix for cosmic rays with energies up to ab ut 10 PeV. Thisγ-ray production matrix is applied to the GeV excess in diffuse Galactic γ-rays observed by EGRET, and we conclude the non-π decay components are insufficient to explain the GeV excess, although they have contributed a different spectrum from theπ-decay component. We also test the hypothesis that the TeV-b andγ-ray emission of the shell-type SNR RX J1713.7-3946 observed wit h HESS is caused by hadronic cosmic rays which are accelerated by a cosmic-ray modified shock. By theχ statistics, we find a continuously softening spectrum is strongly preferred, in contrast to ex pectations. A hardening spectrum has about 1% probability to explain the HESS data, but then only if a har d cutoff at 50-100 TeV is imposed on the particle spectrum. The γ-Ray Production Matrix It is generally believed that diffuse galactic γ-rays are a good probe of the production sites and also the propagation of accelerated charged particles in the Galactic plane (see [1] and references therein). Exact knowledge of theγ-ray source spectrum resulting from hadronic interactions is of prime importance for a proper physical interpretation for the diffuse γ-rays. The production of neutral pions and their subsequent decay to γ-rays is thought to be the principal mechanism for the γ-ray hadronic component in cosmic ray interactions, and a number of parameterizations have been developed to describeγ-ray production inpp-collision [2, 3]. On the other hand, in addition to the neutral pion production, the direct γ-ray production in cosmicray interaction was found important [4], as was the production of(Σ, Σ), (K, K), andη particles, all of which eventually decay into γ-rays [3]. Also, the helium nuclei in cosmic rays and the interstellar medium are expected to contribute about 20-30% to the secondary production in cosmic-ray interactions, for which the multiplicity spectra may be different. In a hadronic interaction, the multiplicity of non-π secondaries withγ-rays as the final decay states is about 50% of the π multiplicity. These nonπ secondaries in hadronic interactions might present a different γ-ray spectral profile. This work presents a careful study of the γ-ray production in cosmic-ray (bothp andα) interactions, by accounting for all decay processes including the direct production. For that purpose we employ the event generator DPMJET-III [5] to simulate secondary productions in both p-generated and He-generated interactions. We include all relevant secondary particles with γ-rays as the final decay products. For the composition of the ISM, we assume 90% protons, 10% helium nuclei, 0.02% carbon, and 0.04% oxygen. Around the energy of π production threshold, where DPMJET appears unreliable, we apply a parametric model [2], that includes the resonance production for the π production. We thus derive aγ-ray production matrix for cosmic rays with energies up to about 10 PeV that can be easily used to interpret the spectra of cosmic γ-ray sources. We consider all the decay channels and their decay fractions published by the Particle Data Group to account for all secondaries (resonances included) Proceedings of the 30th International Cosmic Ray Conference Rogelio Caballero, Juan Carlos D’Olivo, Gustavo Medina-Tanco, Lukas Nellen, Federico A. Sánchez, José F. Valdés-Galicia (eds.) Universidad Nacional Autónoma de México, Mexico City, Mexico, 2008 Vol. 2 (OG part 1), pages 739–742 ID 681