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We investigate the nature of the accelerated particles responsible for theproduction of the gamma-ray emission observed from the middle-aged supernovaremnant (SNR) HB 21. The analysis of more than nine years of Fermi LAT dataleads to the observation of an extended emission positionally in agreement withthe SNR HB 21. The bulk of this gamma-ray emission is detected from theremnant; photons up to $\sim$10\,GeV show clear evidence of curvature at thelower energies. The remnant is characterized by an extension of $0^{\circ}.83$,that is, 30\% smaller than claimed in previous studies. The increasedstatistics allows us also to resolve a point-like source at the edge of theremnant, in proximity to a molecular cloud of the Cyg OB7 complex. In thesouthern part of the remnant, a hint of an additional gamma-ray excess incorrespondence to shocked molecular clouds is observed. The spectral energydistribution of the SNR shows evidence of a break around 400 MeV, which can beproperly fitted within both the hadronic and leptonic scenario. The pion-decaymechanism reproduces well the gamma rays, postulating a proton spectrum with aslope $\sim 2.5$ and with a steepening around tens of GeV, which could beexplained by the energy-dependent escape of particles from the remnant. In theleptonic scenario the electron spectrum within the SNR matches closely thelocally measured spectrum. This remarkable and novel result shows that SNR HB21 could be a direct contributor to the population of Galactic electrons. Inthe leptonic scenario, we find that the local electron spectrum with a breakaround 2 GeV, closely evokes the best-fitting parental spectrum within thisSNR. If such a scenario is confirmed, this would indicate that the SNR might bea source of Galactic background electrons.

Spectral and morphological study of the gamma radiation of the middle-aged supernova remnant HB 21