Detection of the extended $γ$-ray emission from the high Galactic latitude Calvera's SNR candidate

Abstract: We report the extended GeV $\gamma$-ray emission that spatially associated with the high Galactic latitude supernova remnant (SNR) candidate - Calvera's SNR with the Pass 8 data recorded by the {\em Fermi} Large Area Telescope. The $\gamma$-ray spectrum of Calvera's SNR between 100 MeV and 1 TeV shows an evident ($\sim$ 3.4$\sigma$) spectral curvature at several tens of GeV. The multi-wavelength data can be fitted with either a leptonic model or a hadronic one. However, the leptonic model exhibits the inconsistent between the flat radio spectrum and the hard GeV $\gamma$-ray spectrum of Calvera's SNR. For the hadronic model, the spectral index of protons should be harder than 1.6. And the total energy of protons is fitted to be more than one order of magnitude higher than the explosion energy of a typical supernova, which also challenges the hadronic model. The evident spectral curvature and the absence of non-thermal X-ray emission from Calvera's SNR makes it to be an interesting source bridging young-aged SNRs with bright non-thermal X-ray emission and old-aged SNRs interacting with molecular clouds.