Using interstellar clouds to search for Galactic PeVatrons: gamma-ray signatures from supernova remnants

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) = (330.05, 0.13), (15.82, -0.46), (271.09, -1.26), and (21.97, -0.29). 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.

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

“…Table 1 summarises the most promising cloud candidates identified by our study [1]. These were selected as some of the brightest predicted fluxes, more reliable predictions (as a known…”

“…Predictions for -rays from clouds associated with SNR PeVatrons of [1]). Cloud 7, at (32.700, -0.210) is immediately adjacent to LHAASO J1849-0003, one of the recently identified PeVatron sources that could plausibly be associated with either the likely pulsar wind nebula HESS J1849-00 or with the young massive cluster W43 [7].…”

“…Experimental facilities such as Imaging Atmospheric Cherenkov Telescope (IACT) arrays and ground-based particle detectors such as HAWC and LHAASO have in recent years begun to probe the gamma-ray sky above > 100 TeV, providing the first evidence for Galactic PeVatrons. In this contribution, we summarise the key points from the recent modelling study of [1] and discuss these in the context of the PeVatron sources reported by LHAASO [7].…”

The Hunt for Pevatrons: The Case of Supernova Remnants

“…In such a situation, we need alternative ways to probe the past activity of pevatrons, and therefore attempt to do archaeology of pevatrons 1 . The signature of the extinct pevatron could for instance be found through deeper studies of SNR shocks (e.g., extrapolating backwards the shock dynamics), by probing the surrounding molecular clouds (MC) of the pevatron candidates, for instance through gamma rays observations of MCs in the vicinty of pevatron candidates [101][102][103], or through the study of the radiation of secondary particles produced around pevatrons [104].…”

"The ASTRI Mini-Array: a breakthrough in the Cosmic Ray study"

“…However, we have no certainty of the presence of freshly accelerated CRs. The main hypothesis for their hadronic -ray emission are reacceleration of pre-existent CRs [25,26] or suppression of diffusion coefficient close to the sources [27,28].…”