📝 SummarXiv

Abstract

Supernova remnants (SNRs) are widely recognized as key accelerators of Galactic cosmic rays (CRs), supported by the detection of the characteristic pion bump in the gamma-ray spectra of several SNRs. However, the recent observation of ultra-high-energy (UHE, greater than 100 TeV) gamma-rays by LHAASO from sources such as W51 region challenges standard models, which predict CR acceleration up to PeV energies only during the early (~ 100 year) phase of SNR evolution. Given the older age of known SNRs, alternative mechanisms - such as the interaction of runaway CRs with nearby molecular clouds (MCs) - have been proposed to explain the persistent UHE emission. In this study, we focus on the W51 complex, particularly the W51C-B region, as a promising site for investigating SNR-MC interactions. Simulated observations with the CTAO and the ASTRI Mini-Array are presented to demonstrate their crucial role in bridging the energy gap between Fermi-LAT and LHAASO, especially in the 0.3-100 TeV range. Their improved angular resolution will also help disentangle emission components from the interaction zone and nearby sources. Our theoretical modelling suggests that accelerated particles at the shock can account for the radio and GeV data, while UHE emission could be best explained by the combined contribution from both acceleration and adiabatic compression of cloud material at the SNR-MC interface.