HESS J1858+020: A GeV-TeV source possibly powered by CRs from SNR G35.6-0.4

TL;DR

This study investigates the origin of the TeV source HESS J1858+020 near SNR G35.6-0.4, using Fermi-LAT data and hadronic modeling to support the hypothesis that cosmic rays from the SNR produce the observed gamma-ray emission.

Contribution

The paper provides the first detailed analysis linking SNR G35.6-0.4's cosmic rays to the gamma-ray source HESS J1858+020 through combined Fermi-LAT data and hadronic modeling, revealing a lower-than-Galactic diffusion coefficient.

Findings

Identified a hard GeV source coincident with HESS J1858+020 and a molecular cloud complex.

Connected the GeV spectrum of SrcX2 with the TeV spectrum of HESS J1858+020.

Modeled gamma-ray emission with runaway cosmic rays from the SNR, requiring a lower diffusion coefficient.

Abstract

Context: The supernova remnant (SNR) G35.6-0.4 shows a non-thermal radio shell, however, no {\gamma}-ray or X-ray counterparts have been found for it thus far. One TeV source, HESS J1858+020, was found near the SNR and this source is spatially associated with some clouds at 3.6 kpc. Aims: To attain a better understanding of the origin of HESS J1858+020, we further investigate the association between SNR cosmic rays (CRs) and the clouds through the Fermi-LAT analysis and hadronic modeling. Methods: We performed the Fermi-LAT analysis to explore the GeV emission in and around the SNR. We explored the SNR physics with previously observed multi-wavelength data. We built a hadronic model using runaway CRs of the SNR to explain the GeV-TeV observation. Results: We found a hard GeV source (SrcX2) that is spatially coincident with both HESS J1858+020 and a molecular cloud complex at 3.6 kpc. In addition, a soft GeV source (SrcX1) was found at the northern edge of the SNR. The GeV spectrum of SrcX2 connects well with the TeV spectrum of HESS J1858+020. The entire {\gamma}-ray spectrum ranges from several GeV up to tens of TeV and it follows a power-law with an index of ~2.15. We discuss several pieces of observational evidence to support the middle-aged SNR argument. Using runaway CRs from the SNR, our hadronic model explains the GeV-TeV emission at HESS J1858+020, with a diffusion coefficient that is much lower than the Galactic value.