Identifying the counterpart of HESS J1858+020

TL;DR

This study investigates the interstellar medium around HESS J1858+020 and SNR G35.6-0.4, suggesting the gamma-ray emission likely results from hadronic interactions with a nearby molecular cloud, and also considers star formation activity as a possible contributor.

Contribution

It provides the first detailed analysis linking the gamma-ray source HESS J1858+020 to a specific molecular cloud and star formation activity, proposing a hadronic origin for the emission.

Findings

Molecular cloud detected at the SNR border and near HESS J1858+020

Estimated cloud density (~500 cm^{-3}) supports hadronic gamma-ray production

Region shows active star formation, offering alternative emission explanation

Abstract

HESS J1858+020 is a weak gamma-ray source that does not have any clear cataloged counterpart at any wavelengths. Recently, the source G35.6-0.4 was re-identified as a SNR. The HESS source lies towards the southern border of this remnant. The purpose of this work is to investigate the interstellar medium around the mentioned sources in order to look for possible counterparts of the very-high energy emission. Using the 13CO J=1-0 line from the Galactic Ring Survey and mid-IR data from GLIMPSE we analyze the environs of HESS J1858+020 and SNR G35.6-0.4. The 13CO data show the presence of a molecular cloud towards the southern border of SNR G35.6-0.4 and at the same distance as the remnant. This cloud is composed by two molecular clumps, one, over the SNR shell and the other located at the center of HESS J1858+020. We estimate a molecular mass and a density of ~ 5 X 10^{3} Msun and ~ 500 cm^{-3}, respectively for each clump. Considering the gamma-ray flux observed towards HESS J1858+020, we estimate that a molecular cloud with a density of at least 150 cm^{-3} could explain the very-high energy emission hadronically. Thus, we suggest that the gamma-ray emission detected in HESS J1858+020 is due to hadronic mechanism. Additionally, analyzing mid-IR emission, we find that the region is active in star formation, which could be considered as an alternative or complementary possibility to explain the very-high energy emission.