The Contribution of Different Supernova Populations to the Galactic Gamma‐Ray Background

Abstract

The contribution of source cosmic rays (SCRs), accelerated and still confined in supernova remnants (SNRs), to the diffuse high-energy γ-ray emission above 1 GeV from the Galactic disk is studied. The γ-rays produced by SCRs have a much harder spectrum than those generated by Galactic cosmic rays (GCRs). Extending a previous paper, a simple SNR population synthesis is considered and the inverse Compton emission from the SCR electrons is evaluated in greater detail. The combined spectrum of γ-ray emission from the Galactic SNR population is then calculated, and this emission at low Galactic latitudes is compared with the diffuse γ-ray emission observed by EGRET and ground-based instruments. The average contribution of SCRs is comparable to the GCR contribution already at GeV energies, resulting from supernovae of Types II and Ib exploding into the wind bubbles of quite massive progenitor stars, and becomes dominant at γ-ray energies above 100 GeV. At TeV energies, the dominant contribution is from SCRs in SNRs that expand into a uniform interstellar medium. In fact, the sum of hadronic and inverse Compton γ-rays would exceed the limits given by the existing experimental data unless the confinement time TSN, i.e., the time until which SNRs confine the main fraction of accelerated SCRs, is as small as TSN ~ 104 yr and the typical magnetic field strength in SNRs as large as 30 μG. However, both situations are possible as a result of field amplification through cosmic ray (CR) back-reaction in the acceleration process. We point out that accurate measurements of the low-latitude diffuse Galactic γ-ray spectrum at TeV energies can serve as a unique consistency test for CR origin from the SNR population as a whole.