Evidence for the Maximum Energy of Cosmic Rays Accelerated by Supernova Shocks--Purple Mountain Observatory

Evidence for the Maximum Energy of Cosmic Rays Accelerated by Supernova Shocks

Soon after the diffusive shock particle acceleration mechanism was proposed for the origin of Galactic cosmic rays from 1GeV to 1PeV in the 1970s, Lagage and Cesarsky claimed that supernova shocks can at most accelerate cosmic rays to 1 PeV in the most idealized situation.In a more realistic scenario, the shocks of supernova remnants (SNRs) can at most accelerate cosmic rays to 0.1 PeV. Since then, many mechanisms have been proposed to increase the maximum energy of cosmic rays accelerated by supernova shocks to rescue the SNRs scenario for the origin of Galactic cosmic rays.

In a recent paper by ZENG and LIU et al from Purple Mountain Observatory (PMO) of the Chinese Academy of Sciences (CAS), published in The Astrophysical Journal, a sample of 13 SNRs with hard gamma-ray spectra is studied. Such kinds of SNRs are considered the most efficient accelerators of high-energy Galactic cosmic rays. However, this study shows that the distribution of protons producing the observed gamma-ray emission always cuts off below 0.1 PeV, reminiscence of the claim made by Lagage and Cesarky almost 40 year ago. This study therefore gives the first observational evidence for the maximum energy of cosmic rays accelerated by shocks of SNRs, suggesting a class of high energy sources distinct from shocks of SNRs responsible for the acceleration of PeV cosmic rays. Observations of gamma-rays above 0.1 PeV by the Large High Altitude Air-shower Observatory are expected to uncover these sources.

Fig.1 Multiwavelength spectra of 13 SNRs with hard gamma-ray spectra. The dashed line shows a fit to these spectra with a hadronic emission model (Image by ZENG Houdun).