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Gamma-ray emission from young supernova remnants in dense environments

Presentation #102.17 in the session ISM/Galaxies.

Published onJul 01, 2023
Gamma-ray emission from young supernova remnants in dense environments

Supernova remnants are known to accelerate cosmic rays from the detection of non-thermal emission of radio waves, X-rays, and gamma rays. The presence of cut-offs in the gamma-ray spectra of several young SNRs led to the idea that the highest energies might only be achieved during the very initial stages of a remnant’s evolution. Unfortunately, the gamma-ray luminosity is assumed to peak in the first weeks after the Supernova explosion where strong γγ-absorption attenuates the observable signal. Here, we investigate to which extend 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 months to years after the explosion.

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 performed spherically symmetric 1-D simulations in which we simultaneously solve the transport equations for cosmic rays, magnetic turbulence, and the hydrodynamical flow of the thermal plasma in the test-particle limit.

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. The results are compared to calculation of smooth, unstructured winds.

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. As a consequence, the observable flux can be considerably higher compared to the signal expected shortly after the explosion where γγ-absorption is important and where most gamma-ray observatories search for transient signals from these Supernovae. 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.

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