Historical Supernovae

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"On the Jisi day, the 7th day of the month, a big new star appeared in the company of the Ho star."	"On the Xinwei day the new star dwindled."
Figure 1: Bone or tortoise shell inscription from the 14th century BC.

The explosion of a star produces a spectacular astronomical phenomenon -- a supernova. For some time the exploding star becomes as bright as the whole galaxy in which it is embedded. In our own galaxy, the Milky Way, a supernova might even be visible in broad daylight. Only a few supernovae in historical times have been identified in the Milky Way, however, and apparently no star has blown up during the last 300 years. The astronomers are eagerly waiting for the next such event which should reveal to them the secrets of the physics in the center of the star just before the explosion, the explosion mechanism, and the dynamics of the expanding shell of gas. Modern astronomers have to be content with two supernovae in neighbouring galaxies: in the Large Magellanic Cloud in 1987, and in Andromeda in 1895.

Figure 2: The star chart engraved on a stone tablet in Suzhou (1191 AD). The Tianshe region is marked.

More information on historical supernovae in the Milky Way can be gained by an interesting combination of advanced satellite astronomy and ancient Chinese records. The astronomical records of China cover almost 4000 years, and especially changes in the known stellar constellations have been carefully noted. One of the most modern instruments, the satellite ROSAT is devoted to observe the sky in X-rays. This satellite has discovered many extended sources of X-ray emission which bear witness to a supernova explosion - interstellar gas heated up by the shockwave of the supernova, in some cases additional synchrotron radiation perhaps sustained by a pulsar at the center.

Figure 3: A ROSAT X-ray map of the Vela region. The stars Kappa, Delta, Lambda Vel are marked. Two supernova remnants discovered by ROSAT are in the picture. RX J0907-5205 is the remnant which might correspond to the missing star. The upper right hand corner has from the top down two normal stars in X-rays, and the synchrotron nebula surrounding the Vela pulsar.

Can such ROSAT detections of supernova remnants be identified with ancient Chinese records, as in the well-known case of the Crab nebula? Scientists from Munich and Beijing have examined this possibility, and two interesting cases are described here:

1. Inscriptions on bones or tortoise shells from the 14th century BC tell of a supernova event (fig.1). They say that on the Jisi day, the seventh day of the month, a big new star appeared in the company of the Ho star. The Ho star is Antares, and one of the ROSAT supernova remnants, J1714-3939, is close to Antares. This is considered to be a young remnant, because the X-ray spectrum indicates a high temperature. From the picture of a blast wave running out to the observed size with an initial energy typical of a supernova the age is estimated as 3000-5000 years, in agreement with the Chinese record which indicates a supernova about 3500 years ago. This would be the first supernova recorded by human astronomers and identified with a modern instrument.
2. The constellation called "Tianshe" in Chinese, in the constellation Valorum, is represented by 5 bright stars in old star charts such as the Suzhou tablet from 1191 shown in figure 2. In an even earlier catalogue of stars by the Qi state astronomer Gande (about 300 BC) this constellation consisted of 6 stars. The interpretation of the historical texts makes it clear that a star between Delta Vel and Kappa Vel (see fig. 3), but closer to Delta Vel has disappeared sometime between 300 BC and 600 AD. There is a ROSAT source close to that position, J0907-5205. The age is around 1800 years, and this agrees with the Chinese records. The conclusion is obvious: The missing star has exploded as a supernova, and the remnant discovered by ROSAT tells of its fate.

B.Aschenbach (MPE), G.Börner (MPA), Q.B.Li (Beijing Observatory)