VER J2227+608: A Hadronic PeVatron Pulsar Wind Nebula?

Abstract

We report the detection of GeV γ -ray emission from the very-high-energy γ -ray source VER J2227+608 associated with the “tail” region of supernova remnant (SNR) G106.3+2.7. The GeV γ -ray emission is extended and spatially coincident with molecular clouds traced by CO emission. The broadband GeV to TeV emission of VER J2227+608 can be well fitted by a single power-law function with an index of 1.90 ± 0.04, without obvious indication of spectral cutoff toward high energies. The pure leptonic model for the γ -ray emission can be marginally ruled out by the X-ray and TeV data. In the hadronic model, the low energy content of CRs and the hard γ -ray spectrum, in combination with the center bright source structure, suggest that VER J2227+608 may be powered by the Pulsar wind nebula instead of shocks of the SNR. And the cutoff energy of the proton distribution needs to be higher than ∼400 TeV, which makes it an attractive PeVatron candidate. Future observations by the upcoming Large High Altitude Air Shower Observatory and the Cherenkov Telescope Array in the north could distinguish these models and constrain the maximum energy of cosmic rays in SNRs.