G7.7-3.7: A Young Supernova Remnant Probably Associated with the Guest Star in 386 CE (SN 386)

Abstract

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/4{kpc})}^-0.5 {cm}}^-3. A small shock age of 1.2+/- 0.6{(d/4{kpc})}^0.5 kyr is inferred from the low ionization timescale of {2.4}_-1.3^+1.1× {10}¹⁰ {cm}}^-3 {{s}} of the X-ray arc. The low foreground absorption ({N}_{{H}}=3.5+/- 0.5× {10}²¹ {cm}}^-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.