A 鈥済uest star鈥� described in a Chinese journal nearly 2000 years ago has now been confirmed as the world鈥檚 oldest recorded supernova, thanks to X-ray observations from space.
On 7 December 185 AD, Chinese astronomers described the appearance of a 鈥榞uest star鈥� that had 鈥渟cintillating, variegated colours鈥� and grew smaller until it disappeared about eight months later.
But astronomers have debated for more than a dozen years what type of object the guest star actually was. One school of thought contended that it was a comet, as the only supernova remnant in the same area of the sky was estimated to be 10,000 years old 鈥� too old for the supernova that caused it to explain the Chinese sighting.
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But a new look, carried out using the European Space Agency鈥檚 orbiting telescope XMM-Newton and NASA鈥檚 Chandra X-ray Observatory, suggests that this remnant, RCW 86, is exactly the right age to account for the guest star.
鈥淥ur new calculations tell us the remnant is about 2000 years old,鈥� said Aya Bamba of Japan鈥檚 Institute of Physical and Chemical Research (RIKEN), who co-led the study.
Fast expansion
Astronomers arrived at the younger age for the remnant when they realised that its shock wave was expanding faster than previously thought. The discrepancy arose because the shock wave expanded into an irregularly shaped bubble that had been carved out by the star before it exploded as a supernova.
So part of the shock wave had slowed down at the 鈥渆dge鈥� of the bubble when it slammed into surrounding gas there. Now, the X-ray observations reveal sections of the shock wave that are still within the bubble and are therefore still moving at high-speed, allowing researchers to trace the expansion back to a supernova that occurred just about 2000 years ago.
The finding bolsters other evidence that the guest star was a supernova 鈥� such as the fact that the star took about eight months to fade, which is consistent with modern observations of supernovae.
鈥淭here have been previous suggestions that RCW 86 is the remains of the supernova from 185 AD,鈥� said Jacco Vink of the University of Utrecht in the Netherlands. 鈥淭hese new X-ray data greatly strengthen the case.鈥�
