Gamma-ray emission from young supernova remnants in dense environments

Abstract

Supernova remnants are known to accelerate cosmic rays on account of their non-thermal emission in different wavebands. However, evidence that SNRs do indeed accelerate cosmic rays to PeV-energies is elusive. The idea emerged that PeV-energies might only be reached during the very initial stages of a remnant’s evolution. Unfortunately, early gamma-ray emission is strongly attenuated by 𝛾𝛾 -absorption. Here, we investigate how the interaction of SNR-shocks with dense structures in the medium around luminous blue variable (LBV) and Red Supergiant (RSG) stars can boost the gamma-ray emission later. We use the time-dependent acceleration code RATPaC to study the acceleration of cosmic rays in supernovae expanding into dense environments around massive stars. We investigated typical parameters of the circumstellar medium (CSM) in the freely expanding winds and added dense structures that arise from episodes of highly-enhanced mass-loss of LBVs and photoionized shells around RSGs. We find that the interactions with the dense structures happens typically after a few months for LBV progenitors and a few years for RSG progenitors. During the interaction stage, the 𝛾𝛾 -absorption by photons emitted from the Supernova’s photosphere became negligible. The gamma-ray luminosity of the interacting SNRs can surpass the internal/unabsorbed peak-luminosity that arises shortly after the explosion. Further, the change of the shock-speed during the shock-shell interaction boosts the achievable maximum energy beyond a PeV for LBVs, where early interactions yield higher peak-energies. The later