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

¹ IFAE, Edifici C7b, Campus UAB, 08193 Bellaterra, Spain
e-mail: ignasi@ifae.cat
² Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
³ Now at Institut de Ciències de l’Espai (IEEC-CSIC), 08193 Bellaterra, Spain
⁴ Institució Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain

Received: 4 July 2012
Accepted: 22 August 2012

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.