Gamma-ray emission from self-confined cosmic rays around their galactic sources

Abstract

The diffuse γ-ray emission produced by hadronic interaction of cosmic rays (CR) with the interstellar medium (ISM) during their propagation through the Galaxy, reveals important information on the diffusion properties of the ISM as well as on the global distribution of CR sources. In this work we estimate the contribution of γ-ray emission due to the CR self-confined near their acceleration sites. In fact, when CR leave their parent source, they self-generate magnetic turbulence through streaming instability. Such turbulence, in turn, enhances the scattering rate of CRs, increasing their residence time in the region. The streaming instability can be effective within a distance of the order of the coherence scale, Lc, of the background magnetic field from the source, where the density of CRs produced by the source is expected to be larger than the average Galactic value. Nevertheless, the effectiveness strongly dependents on the density of neutral hydrogen, because the ion-neutral friction can suppress the wave amplification. A direct consequence of the self-confinement of CRs is the formation of an extend halo of γ-ray emission of size ∼Lc around each source. The emission from a single halo is too faint to be detected but the sum of these halos over the whole CR source population can be a non negligible contribution to the diffuse gamma-ray emissivity from the Galactic disc. Therefore, we want to compare the predicted emission from the total halo’s population with the diffuse Galactic emission measured by the Fermi-LAT telescope. To do this calculation, we assume that CR sources are Supernova Remnants (SNRs) exploding either in a fully ionized or in a partially ionized ISM. While in the former case the halos emission almost saturate the observed Galactic emission, in the latter, due to the ion-neutral friction, the gamma-ray emission is strongly reduced, being non negligible only for energies of γ-rays Eγ ∼ 100 GeV.