Evidence for hybrid gamma-ray emission from the supernova remnant G150.3+4.5

Abstract: The supernova remnant (SNR) G150.3+4.5 was first identified in radio, exhibiting a hard GeV spectrum and a $\sim 1.5^\circ$ radius. Radio observations revealed a bright arc with an index of $\sim -0.40$, which stands in contrast to the index of $\sim -0.69$ for the rest. This arc is coincident with the point-like \emph{Fermi} source 4FGL J0426.5+5434 and KM2A source 1LHAASO J0428+5531. The rest of the SNR has a hard GeV spectrum and a soft TeV spectrum, implying a spectral cut-off or break near 1 TeV. Since there is no X-ray counterpart and no pulse signal detected, the gamma-ray $(\gamma$-ray) emission mechanism from the SNR and the point-like source appear puzzling. In this work, we reanalyse the $\gamma$-ray emission using 14 yr data recorded by \emph{Fermi} Large Area Telescope and find that the spectrum of the northern half-sphere is compatible with a broken power law with a break at 146 $\pm$ 11 GeV and photon indices of $\Gamma_{\rm{Northlobe}}$ =$1.54\pm0.04_{\rm{stat}}\pm0.07_{\rm{syst}}$ ($2.28\pm0.08_{\rm{stat}}\pm0.12_{\rm{syst}}$) below (above) the break. In addition, the southern half-sphere can be described well with a single power law with $\Gamma_{\rm{Southlobe}}$ =$1.95\pm0.07_{\rm{stat}}\pm0.09_{\rm{syst}}$. Since the southern half-sphere is well correlated with CO emission, we propose that the $\gamma$-ray emission of the northern half-sphere could be dominated by relativistic electrons via inverse-Compton processes, while the southern half-sphere is dominated by cosmic rays via hadronic processes. 4FGL J0426.5+5434 may result from the illumination of a cloud by escaping cosmic rays or recent shock-cloud interaction. Observations from LHAASO-KM2A thus favour the possibility of a cosmic-ray PeVatron candidate, however, leptonic scenarios cannot be ruled out. Further multi-wavelength observations are warranted to confirm the hadronic nature of 1LHAASO J4028+5531.