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

It is difficult to distinguish the 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 this 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 H ii region G35.6-0.5. We reanalyze CO line, H i, and Fermi-LAT GeV gamma-ray emission data of this region. The analysis of the molecular and H i data suggests that SNR G35.6-0.4 and H ii region G35.6-0.5 are located at different distances. The analysis of 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 H ii region G35.6-0.5. The GeV emission of SrcA can be explained by the hadronic process in the SNR-molecular cloud 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 similar to 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 H ii region and should be further verified with ultrahigh-energy observations with, e.g., LHAASO.