SN 1181

First observed between August 4 and August 6, 1181, Chinese and Japanese astronomers recorded the supernova now known as SN 1181 in eight separate texts. One of only five supernovae in the Milky Way confidently identified in pre-telescopic records, it appeared in the constellation Cassiopeia and was visible and motionless against the fixed stars for 185 days. F. R. Stephenson first recognized that the 1181 AD "guest star" must be a supernova, because such a bright transient that lasts for 185 days and does not move in the sky can only be a galactic supernova.

3C 58
Before 2013, the only plausible conventional supernova remnant in the old historical area for the supernova was the supernova remnant 3C 58. This remnant has a radio and X-ray pulsar that rotates about 15 times per second. So historically, SN 1181 had been associated with 3C 58 and its pulsar, although many researchers noted that this association is problematic. For example, if the supernova and pulsar are associated, then the star is still rotating about as quickly as it did when it first formed. This is in contrast to the Crab pulsar, known to be the remnant of the SN 1054 supernova in the year 1054, which has lost two-thirds of its rotational energy in essentially the same span of time.

The age of the 3C 58 remnant has been estimated by many measures. Most directly, the proper motion of the expanding shell of 3C 58 has been measured three times, resulting in a distance-independent estimated age of around 3500 years. The measures of the decline rate of the radio flux have substantial variability and uncertainty, so they are not useful for estimating the remnant's age. Age estimates involving the remnant's energy and the swept-up mass are both not useful due to large uncertainties with the distance as well as the presumed energetics and densities. The pulsar is offset from the center of 3C 58, implying an age of ~3700 years, although it is possible to be substantially younger if its transverse velocity happens to be high. The pulsar spin-down age is 5380 years. The neutron star cooling age is >5000 years. With these age estimates, 3C 58 is much too old a remnant to be associated with SN 1181.

The possible sky position of the 1181 supernova has been revised to include additional information on the proximity of the `guest star' to adjacent Chinese constellations, resulting in a greatly smaller error region. This improved region does not contain 3C 58, because the guest star does not have proximity to two constellations as reported. So SN 1181 is not associated with 3C 58. Rather, this new small region contains Pa 30, which is independently known to be a ~800 year old supernova remnant.

Pa 30
Pa 30 was discovered in 2013 by American amateur astronomer Dana Patchick while searching for planetary nebula in WISE infrared data. It was the 30th nebula discovered by his searches, and as a result it is designated Pa 30. Pa 30 appeared as a nearly-round nebula roughly 171x156 arc-seconds in size, with an extremely blue central star. Pa 30 refers to both the nebula (originally catalogued as IRAS 00500+6713) and the central star (designated as WD J005311). The shell is bright in the infrared, but very faint in the optical, at first visible only by light in the [O III] band.

In 2019, optical spectroscopy of the central star revealed a very hot star with an intense stellar wind expanding at a very high velocity of 16,000 km/s and a composition mainly of carbon, oxygen, and neon (with no hydrogen or helium). Such a speed could only arise from a supernova or an event of similar magnitude, more specifically from a merger of two white dwarfs. X-ray spectroscopy studies of the shell also revealed a very hot nebula containing carbon-burning ashes which can only be produced in a supernova. However, the remnant star of Pa 30 is a white dwarf, not one of the conventional supernova remnants (neutron stars or black holes). It has been suggested that Pa 30 is the remnant of a rare class of supernovae known as "sub-luminous Type Iax Supernova" and that a merger of a CO white dwarf and an ONe white dwarf produced the remnant shell along with its supermassive white dwarf remnant. More recent observations in the [SII] band also revealed fine filamentary structures within the shell that had not previously been seen.

A 2021 study measured the expansion velocity of ~1,100 km/s for the nebula from optical spectroscopy of the [S II] doublet. Together with the angular size of Pa 30 and the GAIA distance of 2.3 kpc, the age of the nebula could be estimated to be approximately 1,000 years. This made Pa 30 the new prime candidate for the remnant of the SN 1181 event. Furthermore, the expansion velocity of the nebula and the inferred absolute brightness of the 1181 event are consistent with a Type Iax Supernova, making Pa 30 the only SN Iax remnant in our Galaxy and the only one which can be studied in detail.

With a temperature near 200,000 K, WD J005311 is the hottest star known. The extreme properties of the central star are being powered by the residual radioactive decay of 56Ni, where the usual half-life of 6.0 days from electron capture is increased to many centuries due to the nickel being completely ionized.