Using Interstellar Clouds to Search for Galactic PeVatrons: Gamma-ray Signatures from Supernova Remnants

TL;DR

This paper identifies promising interstellar cloud and supernova remnant systems as potential galactic PeVatrons by modeling gamma-ray signatures from hadronic cosmic ray interactions, aiding future gamma-ray detection efforts.

Contribution

It provides a systematic characterization of conditions for detectable gamma-ray flux from SNR-cloud systems and ranks the most promising targets for PeV cosmic ray acceleration detection.

Findings

Four clouds with brightest predicted fluxes >100 TeV identified.

Detectability more likely for massive clouds and closer SNR-cloud systems.

Brightest clouds are consistently bright across different model scenarios.

Abstract

Interstellar clouds can act as target material for hadronic cosmic rays; gamma rays subsequently produced through inelastic proton-proton collisions and spatially associated with such clouds can provide a key indicator of efficient particle acceleration. However, even in the case that particle acceleration proceeds up to PeV energies, the system of accelerator and nearby target material must fulfil a specific set of conditions in order to produce a detectable gamma-ray flux. In this study, we rigorously characterise the necessary properties of both cloud and accelerator. By using available Supernova Remnant (SNR) and interstellar cloud catalogues, we produce a ranked shortlist of the most promising target systems, those for which a detectable gamma-ray flux is predicted, in the case that particles are accelerated to PeV energies in a nearby SNR. We discuss detection prospects for future facilities including CTA, LHAASO and SWGO; and compare our predictions with known gamma-ray sources. The four interstellar clouds with the brightest predicted fluxes >100 TeV identified by this model are located at (l,b) = (333.46,-0.31), (16.97,0.53), (110.43,1.89) and (336.73,-0.98). These clouds are consistently bright under a range of model scenarios, including variation in the diffusion coefficient and particle spectrum. On average, a detectable gamma-ray flux is more likely for more massive clouds; systems with lower separation distance between the SNR and cloud; and for slightly older SNRs.