Predictions for gamma-rays from clouds associated with supernova remnant PeVatrons

TL;DR

This paper assesses the potential for gamma-ray detection from interstellar clouds associated with supernova remnants (SNRs) acting as PeV cosmic-ray accelerators, identifying promising targets and detection prospects for future gamma-ray observatories.

Contribution

It characterizes the conditions for detectable gamma-ray flux from clouds near PeVatron SNRs and ranks promising target systems using catalog data and modeling.

Findings

Detectable gamma-ray flux more likely from more massive clouds.

Closer SNR-cloud separation increases detection prospects.

Older SNRs provide more favorable conditions for gamma-ray detection.

Abstract

Interstellar clouds can act as target material for hadronic cosmic rays; gamma-rays produced through inelastic proton-proton collisions and spatially associated with the clouds provide a key indicator of efficient particle acceleration. However, even for PeVatron sources reaching PeV energies, the system of cloud and accelerator must fulfil several conditions in order to produce a detectable gamma-ray flux. In this contribution, we characterise the necessary properties of both cloud and accelerator. Using available Supernova Remnant (SNR) and interstellar cloud catalogues, and assuming particle acceleration to PeV energies in a nearby SNR, we produce a ranked shortlist of the most promising target systems; those for which a detectable gamma-ray flux is predicted. We discuss detection prospects for future facilities including CTA and SWGO; and compare our predictions with known gamma-ray sources, including the Ultra-High-Energy sources recently detected by LHAASO. A range of model scenarios are tested, including variation in the diffusion coefficient and particle spectrum, under which the best candidate clouds in our shortlist are consistently bright. On average, a detectable gamma-ray flux is more likely for more massive clouds; for systems with lower separation distance between the SNR and cloud; and for slightly older SNRs, due to the time required for particles to traverse the separation distance.