LHAASO\,J2108+5157 as a molecular cloud illuminated by a supernova remnant

Abstract

The search for Galactic PeVatrons - astrophysical accelerators of cosmic rays to PeV energies - has entered a new phase in recent years with the discovery of the first ultra-high-energy (UHE, $E>100$\,TeV) gamma -ray sources by the High Altitude Water Cherenkov (HAWC) observatory and Large High Altitude Air Shower Observatory (LHAASO). Establishing whether the emission is leptonic or hadronic in nature, however, requires multi-wavelength data and modelling studies. Among the currently known UHE sources, LHAASO\,J2108+5157 is an enigmatic source without clear association to a plausible accelerator, yet spatially coincident with molecular clouds. We aim to investigate the scenario of a molecular cloud illuminated by cosmic rays accelerated in a nearby supernova remnant (SNR) as an explanation for LHAASO\,J2108+5157. We aim to constrain the required properties of the SNR as well as which of the clouds identified in the vicinity is the most likely association. We used a model for cosmic-ray acceleration in SNRs, their transport through the interstellar medium, and subsequent interaction with molecular material to predict the corresponding gamma -ray emission. The parameter space of SNR properties was explored to find the most plausible parameter combination that can account for the gamma -ray spectrum of LHAASO\,J2108+5157. In the case that a SNR is illuminating the cloud, we find that it must be young ($<10$\,kyr) and located within $40-60$\,pc of the cloud. A SN scenario with a low Sedov time is preferred, with a maximum proton energy of 3\,PeV assumed. No SNRs matching these properties are currently known, although an as yet undetected SNR remains feasible. The Galactic CR sea is insufficient to solely account for the observed flux, such that a PeVatron accelerator must be present in the vicinity.