The Wěi asterism appears in the lower half of the Scorpius constellation
|Supernova type||Type II/Ib|
|Host galaxy||Milky Way|
|Right ascension||17h 14m|
|Discovery date||393 CE|
|Peak magnitude (V)||−1|
|Distance||1 kpc (3 kly)|
The second lunar month mentioned in the record corresponds to the period February 27 to March 28, 393 CE, while the ninth lunar month ran from October 22 to November 19, 393 CE. The bowl-shaped asterism named Wěi is formed by the tail of the modern constellation Scorpius. This asterism consists of the stars in Scorpius designated ε, μ, ζ, η, θ, ι, κ, λ and ν. The guest star reached an estimated apparent magnitude of −1 and was visible for about eight months before fading from sight. The duration of this event suggests the source was a supernova, which is a cataclysmic explosion of a star.
When the material from a supernova explosion is ejected from the star, it sweeps up the surrounding interstellar material and creates an expanding supernova remnant of gas and plasma. In 1975, there were only seven known supernova remnants in the region of the sky where SN 393 was observed. On the basis of the peak magnitude, supernova SN 393 was initially estimated to have occurred at a distance close to 10 kpc (33 kly). This requirement ruled out all but three of the candidates. Of the remainder, the first, G350.0-1.8 has an estimated age of 8,000 years; too old to be the remnant of SN 393. The two remaining sources, G348.5+0.1 and G348.7+0.3 are located at a suitable distance of about 10 kpc and have an estimated age of 1,500 years. However, because this supernova occurred along the dusty galactic plane, it is difficult to explain how it could have been observed with the naked eye from a distance of 10 kpc for a period of eight months.
In 1996, the ROSAT All Sky Survey discovered a new supernova remnant in this area of the sky: RX J1713.7-3946. Two years later, it was suggested that this might be a better match for SN 393. Observations in 1999 suggested that this remnant is associated with the H II region G347.611 +0.204, which is at a distance of 6 kpc (20 kly). However, in 2003, examination of the interaction between this cloud and a molecular cloud resulted in a closer distance of 1 kpc (3 kly). This estimate was strengthened by a 2004 study of the X-ray and neutral hydrogen absorption by matter between the remnant and the Earth. Given the remnant's angular size of 70 arcminutes on the sky, at this distance it has a physical diameter of 20 pc (65 ly).
The supernova remnant RX J1713.7-3946 is consistent with a type II or type Ib supernova explosion of a star with an initial mass at least 15 solar masses. The explosion generated an energy of about 1.3 × 1051 erg, ejecting three solar masses of material into the surrounding interstellar medium.
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