2018ApJ...865L...6Z

Although the Galactic supernova rate is about two per century, only few supernova remnants (SNRs) are associated with historical records. There are a few ancient Chinese records of "guest stars" that are probably sightings of supernovae for which the associated SNRs are not established. Here we present an X-ray study of the SNR G7.7-3.7, as observed by XMM-Newton, and discuss its probable association with the guest star of 386 CE. This guest star occurred in the ancient Chinese asterism Nan-Dou, which is part of Sagittarius. The X-ray morphology of G7.7-3.7 shows an arc-like feature in the SNR south, which is characterized by an under-ionized plasma with subsolar abundances, a temperature of 0.4-0.8 keV, and a density of ∼0.5(d/4kpc)^–0.5cm^–3. A small shock age of 1.2±0.6(d/4kpc)^0.5 kyr is inferred from the low ionization timescale of 2.4_–1.3^+1.1×10¹⁰cm^–3s of the X-ray arc. The low foreground absorption (N_H=3.5±0.5×10²¹cm^–2) of G7.7-3.7 made the supernova explosion visible to the naked eye on the Earth. The position of G7.7-3.7 is consistent with the event of 386 CE, and the X-ray properties suggest that its age is also consistent. Interestingly, the association between G7.7-3.7 and guest star 386 suggests that the supernova was a low-luminosity supernova, thus explaining the brief visibility (2-4 months) of the guest star.