Gamma rays from supernova remnants

The recent galactic plane surveys with space- and ground-based detectors revealed a number of high and very-high energy γ-ray sources associated with young and middle-aged supernova remnants (SNRs). These results imply effective production of relativistic particles, most likely through the process of diffusive shock acceleration. The interpretation of γ-ray data from several prominent representatives of young SNRs within the so-called ‘hadronic models’ demands hard proton spectra extending to 100 TeV, and total energy released in accelerated protons and ions W CR ⩾ 10 50 erg. This can be treated as a support of the SNR paradigm of galactic cosmic rays. However, the hadronic models are not free of pitfalls, and pose in fact non-trivial challenges. Moreover, in many cases γ-ray data can be successfully explained also by the inverse Compton scattering of directly accelerated electrons. Future deep spectroscopic and morphological studies of SNRs with the planned Cherenkov Telescope Array (CTA) over four decades in energy, from approximately 30 GeV to 300 TeV, promise a breakthrough regarding the identification of radiation mechanisms. The extension of studies beyond the shells of SNRs will be another important objective aimed at the extraction of information about the highest energy particles which have left remnants at the early epochs of their evolution. These particles, which potentially carry an answer to a key question whether SNRs operate as PeVatrons, can be traced via γ-ray emission outside the remnants. Finally, the expected significant increase of the number of γ-ray emitting SNRs by CTA should allow compelling population studies – a key issue for the proof of the SNR origin of galactic cosmic rays.