Speaker
Description
SNR G150.3+4.5 was first identified in radio and has a hard overall GeV spectrum with $\sim 1.5^\circ$ radius. Radio observations have revealed a bright hard arc with an index of $\sim -0.40$ in contrast to the index of $\sim -0.69$ for the other part. This arc is coincident with the soft Fermi source 4FGL J0426.5+5434 (Hereafter SrcX) and the soft ultra-high-energy (UHE) source 1LHAASO J0428+5531 discovered by LHAASO-KM2A. The rest of the SNR however has a hard GeV spectrum and a soft TeV spectrum, implying a spectral cutoff or break near 1 TeV. Since there is no X-ray counterpart and no pulse signal has been detected from SrcX, the nature of SNR G150.3+4.5 is quite puzzling. We reanalyze its $\gamma$-ray emission using 14 yr of Pass 8 data recorded by the Fermi Large Area Telescope and find that the spectra of the northern and southern half-spheres are compatible with single power-laws with indexes $\Gamma_{\rm{SrcN}} \sim$$1.56\pm0.11$ and $\Gamma_{\rm{SrcS}} \sim$$1.98\pm0.07$, respectively. Since the southern half-sphere is well correlated with CO emission, we propose that the $\gamma$-ray emission of the northern half-sphere is dominated by relativistic electrons via the inverse-Compton processes, while that of the southern half-sphere is dominated by cosmic rays via the hadronic processes. SrcX can result from illumination of a cloud by escaping cosmic rays or recent shock-cloud interaction. LHAASO observations of SNR G150.3+4.5 therefore indicate that it is likely a cosmic ray PeVatron. Further multi-wavelength observation of SrcX are warranted to confirm its nature.
