Evidence for protons accelerated and escaped from the Puppis A region using Fermi-LAT observations

Supernova remnants (SNRs) interacting with molecular clouds are interesting laboratories for studying the acceleration of cosmic rays and their propagation in the dense ambient medium. We analyzed 14 years of Fermi-LAT observations of the supernova remnant Puppis A to investigate its asymmetric γ-ray morphology and spectral properties. This middle-aged remnant (∼4 kyr) is evolving in an inhomogeneous environment, interacting with a dense molecular cloud in the northeast and a lower-density medium in the southwest. We find clear differences in both γ-ray luminosity and spectral energy distribution between these two regions. The emission from both sides is consistent with a hadronic origin. However, whereas the southwestern emission can be explained by standard diffusive shock acceleration (DSA), the northeastern side may involve reacceleration of preexisting cosmic rays or acceleration via reflected shocks in the dense cloud environment. Additionally, we identify two significant γ-ray excesses outside the remnant, including a previously unreported source to the south. These features are likely produced by cosmic rays that have escaped Puppis A and are interacting with nearby dense molecular material. From this extended emission, we estimate the total energy in escaping cosmic rays to be W_ CR ∼ 1.5 erg, providing important constraints on cosmic-ray propagation around the remnant.