Gamma-ray emitting supernova remnants as the origin of Galactic cosmic rays?

Abstract

The origin of cosmic rays is one of the long-standing mysteries in physics and astrophysics. Simple arguments suggest that a scenario of supernova remnants (SNRs) in the Milky Way as the dominant sources for the cosmic ray population below the knee could work: in a generic calculation, it can be shown that these objects can provide the energy budget necessary to explain the observed flux of cosmic rays. However, this argument is based on the assumption that all sources behave in the same way, i.e.\ they all have the same energy budget, spectral behavior and maximum energy. In this paper, we investigate if a realistic population of SNRs is capable of producing the cosmic ray flux as it is observed below the knee. We use 21 SNRs that are well-studied from radio wavelengths up to gamma-ray energies. It could be shown previously (Mandelartz & Becker Tjus 2015) that the high-energy bump in the energy spectrum of these 21 sources can be dominated by hadronic emission. Here, gamma-rays are produced via $π^{0}-$decays from cosmic ray interactions in molecular clouds near the supernova remnant, which serves as the cosmic ray accelerator. The cosmic ray spectra show a large variety in their energy budget, spectral behavior and maximum energy. These sources are assumed to be representative for the total class of SNRs, where we assume that about 100 - 200 cosmic ray emitting SNRs should be present today. Finally, we use these source spectra to simulate the cosmic ray transport from individual SNRs in the Galaxy with the GALPROP code for cosmic ray propagation. We find that the cosmic ray budget can be matched well for a diffusion coefficient that is close to $D\propto E^{0.3}$. A stronger dependence on the energy, e.g. $E^{0.5}$, would lead to a spectrum at Earth that is too steep when compared to what is detected and the energy budget cannot be matched, in particular toward high energies.