Abstract
Aims. We investigate the nature of the accelerated particles responsible for the production of the gamma-ray emission observed from the middle-aged supernova remnant (SNR) HB 21. Methods. We present the analysis of more than nine years of Fermi LAT data from the SNR HB 21. We performed morphological and spectral analysis of the SNR by means of a three-dimensional binned likelihood analysis. To assess the intrinsic properties of the parent particle models, we fit the obtained gamma-ray spectral energy distribution of the SNR by both hadronic- and leptonic-induced gamma-ray spectrum. Results. We observe an extended emission positionally in agreement with the SNR HB 21. The bulk of this gamma-ray emission is detected from the remnant; photons up to ~10 GeV show clear evidence of curvature at the lower energies. The remnant is characterized by an extension of 0°.83, that is, 30% smaller than claimed in previous studies. The increased statistics allowed us also to resolve a point-like source at the edge of the remnant, in proximity to a molecular cloud of the Cyg OB7 complex. In the southern part of the remnant, a hint of an additional gamma-ray excess in correspondence to shocked molecular clouds is observed. Conclusions. The spectral energy distribution of the SNR shows evidence of a break around 400 MeV, which can be properly fitted within both the hadronic and leptonic scenario. The pion-decay mechanism reproduces well the gamma rays, postulating a proton spectrum with a slope ~2.5 and with a steepening around tens of GeV, which could be explained by the energy-dependent escape of particles from the remnant. In the leptonic scenario the electron spectrum within the SNR matches closely the locally measured spectrum. This remarkable and novel result shows that SNR HB 21 could be a direct contributor to the population of Galactic electrons. In the leptonic scenario, we find that the local electron spectrum with a break around 2 GeV, closely evokes the best-fitting parental spectrum within this SNR. If such a scenario is confirmed, this would indicate that the SNR might be a source of Galactic background electrons.
Highlights
Supernova remnants (SNRs) are believed to be sources of Galactic cosmic rays (CRs)
Several scenarios describing the morphology of HB 21 were considered and tested as follows: 1. a disk-type geometry with fixed radius of 1◦.19, as estimated in Pivato et al (2013); 2. a disk-type geometry with free radius; 3. a radial Gaussian shape with free width; 4. a ring-type geometry defined on the basis of the radio contours of the supernova remnant (SNR) at 4850 MHz, i.e., an elliptical ring 40◦ tilted in the northwest-southeast direction with inner radii of 0◦.3 and 0◦.45 and outer radii of 0◦.9 and 1◦
We analyzed more than nine years of Fermi-LAT Pass-8 data from the region of the SNR HB 21
Summary
Supernova remnants (SNRs) are believed to be sources of Galactic cosmic rays (CRs). According to the SNR paradigm, CRs are accelerated through diffusive shock acceleration (DSA) at SNR shocks and propagate diffusively in the Galaxy (Bell 1978; Blandford & Ostriker 1978). At the time of writing, this spectral feature is claimed to be detected in three bright SNRs: IC 443 (Ackermann et al 2013), W44 (Giuliani et al.2011; Ackermann et al 2013), and W51C (Jogler & Funk 2016). All of these three objects are middle-aged SNRs interacting with molecular clouds, characterized by a shell-like radio morphology combined with a center-filled X-ray morphology
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