An extended source of GeV gamma rays coincident with the supernova remnant HB 21

TL;DR

This study analyzes 3.5 years of Fermi/LAT data to identify an extended gamma-ray source associated with supernova remnant HB 21, revealing complex morphology and spectral features linked to molecular clouds and the remnant shell.

Contribution

It provides the first detailed modeling of HB 21's gamma-ray emission as an extended source with regional spectral differences, linking gamma-ray features to molecular clouds and remnant structure.

Findings

Gamma-ray emission is from a single extended source modeled as a uniform circle.

The spectral energy distribution peaks at 413 MeV with a curved power law.

One region shows a softer spectrum and is coincident with the remnant shell.

Abstract

We analyze 3.5 years of public Fermi/LAT data around the position of the supernova remnant HB 21, where four point-like sources from the 2nd Fermi/LAT catalog are located. We determine that the gamma-ray source is produced by a single extended source. We model the observed morphology as a uniform circle. The spectral energy distribution is best described by a curved power law, with a maximum at 413+-11 MeV. We divide the circle into three regions defined by previously identified shocked molecular clouds, and find that one of these regions has a softer spectrum. The >3 GeV gamma-ray emission of the soft spectrum region is bow-shaped and coincident with the supernova remnant shell seen at radio wavelengths. We suggest that the gamma-ray emission from HB 21 can be understood as a combination of emission from shocked/illuminated molecular clouds, one of them coincident with the supernova remnant shell itself.