GeV Gamma-ray Emission and Molecular Clouds towards Supernova Remnant G35.6$-$0.4 and the TeV Source HESS J1858+020

TL;DR

This study reanalyzes gamma-ray data around SNR G35.6-0.4 and HESS J1858+020, revealing potential hadronic processes and a PeVatron, advancing understanding of cosmic ray origins.

Contribution

It identifies distinct gamma-ray sources and demonstrates a hadronic origin for the emission, suggesting the presence of a PeVatron in the HII region.

Findings

GeV emission from two sources, SrcA and SrcB, with SrcA linked to the SNR.

The spectrum of SrcB and HESS J1858+020 can be connected by a power-law, indicating hadronic origin.

Evidence for a proton cutoff energy above 1 PeV, implying a PeVatron.

Abstract

It is difficult to distinguish hadronic process from the leptonic one in $\gamma$-ray observation, which is however crucial in revealing the origin of cosmic rays. As an endeavor in the regard, we focus in this work on the complex $\gamma$-ray emitting region, which partially overlaps with the unidentified TeV source HESS~J1858+020 and includes supernova remnant (SNR) G35.6$-$0.4 and HII region G35.6$-$0.5. We reanalyze CO-line, HI, and Fermi-LAT GeV $\gamma$-ray emission data of this region. The analysis of the molecular and HI data suggests that SNR G35.6$-$0.4 and HII region G35.6$-$0.5 are located at different distances. The analysis the GeV $\gamma$-rays shows that GeV emission arises from two point sources: one (SrcA) coincident with the SNR, and the other (SrcB) coincident with both HESS J1858+020 and HII region G35.6$-$0.5. The GeV emission of SrcA can be explained by the hadronic process in the SNR-MC association scenario. The GeV-band spectrum of SrcB and the TeV-band spectrum of HESS J1858+020 can be smoothly connected by a power-law function, with an index of $\sim$2.2. The connected spectrum is well explained with a hadronic emission, with the cutoff energy of protons above 1 PeV. It thus indicates that there is a potential PeVatron in the HII region and should be further verified with ultra-high energy observations with, e.g., LHAASO.