Population study of Galactic supernova remnants at very high $\gamma$-ray energies with H.E.S.S.

TL;DR

This study analyzes the population of Galactic supernova remnants at very high gamma-ray energies using H.E.S.S. data, providing flux upper limits and constraining physical parameters, supporting the cosmic ray acceleration paradigm.

Contribution

It offers a comprehensive VHE gamma-ray survey of SNRs, deriving flux upper limits for undetected remnants and constraining ambient density and electron-proton energy fractions.

Findings

Flux upper limits are consistent with cosmic ray acceleration models.

Constraints on ambient density around SNRs are established.

Evidence supports magnetic field amplification in shell-type SNRs.

Abstract

Shell-type supernova remnants (SNRs) are considered prime candidates for the acceleration of Galactic cosmic rays (CRs) up to the knee of the CR spectrum at $\mathrm{E} \approx \mathrm{3}\times \mathrm{10}^\mathrm{15}$ eV. Our Milky Way galaxy hosts more than 350 SNRs discovered at radio wavelengths and at high energies, of which 220 fall into the H.E.S.S. Galactic Plane Survey (HGPS) region. Of those, only 50 SNRs are coincident with a H.E.S.S source and in 8 cases the very high-energy (VHE) emission is firmly identified as an SNR. The H.E.S.S. GPS provides us with a legacy for SNR population study in VHE $\gamma$-rays and we use this rich data set to extract VHE flux upper limits from all undetected SNRs. Overall, the derived flux upper limits are not in contradiction with the canonical CR paradigm. Assuming this paradigm holds true, we can constrain typical ambient density values around shell-type SNRs to $n\leq 7~\textrm{cm}^\textrm{-3}$ and electron-to-proton energy fractions above 10~TeV to $\epsilon_\textrm{ep} \leq 5\times 10^{-3}$. Furthermore, comparisons of VHE with radio luminosities in non-interacting SNRs reveal a behaviour that is in agreement with the theory of magnetic field amplification at shell-type SNRs.