Exploring the supernova remnant contribution to the first LHAASO source catalog via passively illuminated interstellar clouds

TL;DR

This paper investigates whether supernova remnants interacting with interstellar clouds could explain some of the ultra-high energy gamma-ray sources detected by LHAASO, suggesting a potential contribution of passive gamma-ray emission from such interactions.

Contribution

It introduces a physically driven model linking SNRs, cosmic-ray escape, and molecular clouds to explain LHAASO sources, highlighting the role of passive gamma-ray emission.

Findings

Interstellar clouds illuminated by SNRs may contribute to gamma-ray flux.

Further detailed studies are needed to confirm this scenario.

Some unidentified LHAASO sources could be explained by SNR-cloud interactions.

Abstract

Supernova remnants (SNRs) are considered as the most promising source class to account for the bulk of the Galactic cosmic-ray flux. Yet amongst the population of ultra-high energy (UHE) sources that has recently emerged, due to high-altitude particle detector experiments such as LHAASO and HAWC, remarkably few are associated with known SNRs. These observations might well indicate that the highest energy particles would escape the remnant early during the shock evolution as a result of its reduced confinement capabilities. This flux of escaping particles may then encounter dense targets (gas and dust) for hadronic interactions in the form of both atomic and molecular material such as interstellar clouds, thereby generating a UHE gamma-ray flux. We explore such a scenario here, considering known SNRs in a physically driven model for particle escape, and as coupled to molecular clouds in the Galaxy. Our analysis allows the investigation of SNR-illuminated clouds in coincidence with sources detected in the first LHAASO catalogue. Indeed, the illuminated interstellar clouds may contribute to the total gamma-ray flux from several unidentified sources, as we discuss here. Yet we nevertheless find that further detailed studies will be necessary to verify or refute this scenario of passive UHE gamma-ray sources in future.