High energy diffuse gamma-ray emission of the galactic disk and galactic cosmic-ray spectra

Abstract

Observations of diffuse Galactic γ-ray spectrum by the EGRET instrument reveal an excess above ∼1 GeV over the expected γ-ray spectrum calculated under the assumption that the locally observed cosmic-ray (CR) spectra represent the galactic CR spectra. Assuming that Galactic CRs of energy below ∼100 TeV are accelerated by supernova remnant (SNR) shock waves and that the shock compression ratio is SNR age dependent, the average source injection spectra from an ensemble of SNRs is calculated both in the inner (330° < l < 30°) and outer (30° < l < 330°) regions of the galaxy. The calculation considers the SNR age distribution in the galaxy. Injecting these spectra in the galaxy and using a 3-D convection–diffusion equation, the CR electrons and protons spectra in the two galactic regions are obtained and their spectra in the galactic disk are found to be flatter than the observed CR spectra. The diffuse gamma-ray spectrum produced by the interaction of these galactic CRs with the ISM and ISRFs is compared with the experimental data in both the galactic regions. Furthermore, the steepening of the observed local CR spectra from the galactic disk CR spectra are discussed by propagating local CRs having a source spectrum derived using local SNR age distribution (SNRs located within 1.5 kpc from the Sun), for a diffusion coefficient D 0 ∼ 0.3 × 10 27 cm 2 s −1 in the local region which is much less than the typical value in the galaxy D 0 ∼ (1–10) × 10 28 cm 2 s −1. The results obtained in this paper support the SNR origin of galactic CRs.