Gamma-ray emission of supernova remnants and the origin of galactic cosmic rays

Abstract

The recent surveys of the Milky Way with space and ground-based gamma-ray detectors revealed hundreds of high energy (HE) and tens of very high energy (VHE) gamma-ray emitters representing several galactic source populations - supernova remnants, giant molecular clouds, star forming regions, pulsars, pulsar wind nebulae, binary systems. The major fraction of these objects remains however unidentified. In this chapter I discuss the astrophysical implications of VHE gamma-ray observations of supernova remnants (SNRs) in the context of the origin of galactic cosmic rays. These observations confirm the earlier theoretical predictions of effective acceleration of multi-TeV particles in young SNRs by strong shock waves. The interpretation of VHE gamma-ray data from several prominent representatives of young SNRs within the so-called hadronic models requires hard energy spectra of protons extending to 100 TeV, with total energy released in relativistic protons and nuclei as large as 10 50 erg. Formally, this can be considered as an observational proof of the so-called SNR paradigm of the origin of galactic cosmic rays. However, the hadronic models are not free of problems related to interpretation of multi-wavelength properties of these objects. Moreover, in most of the cases the gamma-ray data can be explained by the inverse Compton scattering of electrons which are responsible also for the synchrotron X-radiation of young SNRs. These circumstances prevent us from a firm statement about the contribution of SNRs to the overall flux of galactic cosmic rays. Further observations of young SNRs, especially in the highest energy band (well above 10 TeV), can be crucial in this regard. Quite important are also the complementary observations from massive molecular clouds located within the close proximity of mid-age SNRs.