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We present the analysis of Fermi Large Area Telescope (LAT) {\gamma}-rayobservations of HB 21, a mixed-morphology supernova remnant. Such supernovaremnants are characterized by an interior thermal X-ray plasma, surrounded by awider nonthermal shell emitting at radio frequencies. HB 21 has a large angularsize, making it a good candidate for detailed morphological and spectralstudies with the LAT. The radio extension is 2{\deg}x1{\deg}.5, compared to theLAT 68% containment angle of ~1{\deg} at 1 GeV. To understand the origin of{\gamma}-ray emission, we compare LAT observations with other wavelengths thattrace non-thermal radio synchrotron, nearby molecular clouds, shocked molecularclumps, and the central X-ray plasma. Finally, we model possible hadronic andleptonic emission mechanisms. We conclude that {\gamma}-rays from HB 21 arelikely the result of electron bremsstrahlung or proton-proton collisions withdense material due to interaction with the nearby clouds.

Fermi LAT gamma-ray observations of the supernova remnant HB21