Fermi LAT gamma-ray observations of the supernova remnant HB21

TL;DR

This paper analyzes Fermi LAT gamma-ray data of the supernova remnant HB 21, revealing that its gamma-ray emission likely originates from interactions with nearby molecular clouds, through either leptonic or hadronic processes.

Contribution

First detailed gamma-ray morphological and spectral analysis of HB 21, linking gamma-ray emission to interactions with molecular clouds and modeling possible emission mechanisms.

Findings

Gamma-ray emission is consistent with electron bremsstrahlung or proton-proton collisions.

Emission correlates with molecular clouds and shocked regions.

Supports hadronic or leptonic origin of gamma rays in HB 21.

Abstract

We present the analysis of Fermi Large Area Telescope (LAT) {\gamma}-ray observations of HB 21, a mixed-morphology supernova remnant. Such supernova remnants are characterized by an interior thermal X-ray plasma, surrounded by a wider nonthermal shell emitting at radio frequencies. HB 21 has a large angular size, making it a good candidate for detailed morphological and spectral studies with the LAT. The radio extension is 2{\deg}x1{\deg}.5, compared to the LAT 68% containment angle of ~1{\deg} at 1 GeV. To understand the origin of {\gamma}-ray emission, we compare LAT observations with other wavelengths that trace non-thermal radio synchrotron, nearby molecular clouds, shocked molecular clumps, and the central X-ray plasma. Finally, we model possible hadronic and leptonic emission mechanisms. We conclude that {\gamma}-rays from HB 21 are likely the result of electron bremsstrahlung or proton-proton collisions with dense material due to interaction with the nearby clouds.