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

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