G. Pivato, J. W. Hewitt, L. Tibaldo
We present the analysis of Fermi Large Area Telescope (LAT) \g-ray observations of HB 21, a mixed-morphology supernova remnant. Such supernova remnants are characterized by an interior thermal X-ray plasma, surrounded by a wider nonthermal shell emitting at radio frequencies. HB 21 has a large angular size, making it a good candidate for detailed morphological and spectral studies with the LAT. The radio extension is $2^\circ\times1^\circ$, compared to the LAT 68% containment angle of $\sim1^\circ$ at 1 GeV. To understand the origin of \g-ray emission, we compare LAT observations with other wavelengths that trace non-thermal radio synchrotron, nearby molecular clouds, shocked molecular clumps, and the central X-ray plasma. Finally, we model possible hadronic and leptonic emission mechanisms. We conclude that \g-rays from HB 21 are likely the result of electron bremsstrahlung or proton-proton collisions with dense material due to interaction with the nearby clouds.
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http://arxiv.org/abs/1303.2091
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