Reassessing Galactic Cosmic-Ray Sources Using GeV–PeV Gamma-Ray Spectra

Abstract

Cosmic rays (Crs) with energies up to a few tens of PeV are generally believed to originate within our galaxy, while those exceeding a few EeV are thought to have extragalactic origins. Supernova remnants (SNRs) are widely considered the most likely sources of galactic CRs. Detecting high-energy gamma rays and neutrinos with appropriate flux levels from potential astrophysical sources could provide definitive evidence linking them to CR production. In this study, we critically evaluate the SNR paradigm in light of recent high-energy gamma-ray observations. Specifically, we examine the spectral indices of gamma-ray emissions from various SNRs based on multiple observations. Our key findings include: i) Not all SNRs function as PeVatrons, as their maximum CR energies fall below the expected knee energy. ii) The spectral indices of gamma-ray emissions from different SNRs vary significantly, contradicting theoretical predictions. Additionally, observations from the Large High Altitude Air Shower Observatory (LHAASO) suggest that several pulsars may act as CR PeVatrons. We also investigate the potential of pulsars as primary CR sources within our galaxy and find that PeV gamma-ray spectra from both SNRs and pulsars are softer than their GeV and TeV counterparts. These findings challenge the widely accepted SNR paradigm as the primary source of galactic CRs. Furthermore, the pulsars detected at PeV energies by LHAASO are also unlikely to be the dominant sources of galactic CRs.