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

¹ School of Physics and Astronomy, Sun Yat-Sen University, Guangzhou 510275, PR China
e-mail: cuiyd@mail.sysu.edu.cn; tanbxuan@mail.sysu.edu.cn
² School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031, PR China
e-mail: ylxin@swjtu.edu.cn
³ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210033, PR China
⁴ Institut für Astronomie und Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China

Received: 27 July 2020
Accepted: 19 December 2020

Abstract

Context. The supernova remnant (SNR) G35.6−0.4 shows a non-thermal radio shell, however, no γ-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 γ-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.