Supernova Cassiopeia A is one of the best-studied supernova remnants in the Milky Way. From our point of view, the shell of plasma has been expanding for 350 years at an average rate of between 4,000 and 6,000 kilometers per second. But not all of it. An area of the supernova appears to be going backwards.
The discovery, accepted for publication in the Astrophysical Journal and available as a preprint on the ArXiv, is based on 19 years’ worth of data that suggests that the western part of the remnant has slammed into something. An alternative explanation is that there is a hole in the shell, but models favor the collision scenario.
"The backward movement in the west can mean two things," lead author Jacco Vink, from the University of Amsterdam, said in a statement. "Either there is a hole somewhere, a kind of vacuum, in the supernova material, causing the hot shell to suddenly move inwards locally. Or the nebula has collided with something."
The observations were conducted using NASA’s Chandra X-ray Observatory. The researchers measured the acceleration (or deceleration) of the inner and outer shells of the supernova. When Vink first reported these findings in 2019 at a specialist conference, it attracted the attention of an Italian group of astronomers. Intrigued they set out to simulate what might be happening in the event of a collision.
As a star gets into the red giant phase and subsequently in its super red giant phase, it blows a lot of material away. These stars are enormous and their outer layers are loosely bound. Once the star goes supernova — the cataclysmic explosion at the end of its life — the material is thrown at high-speed and it can catch up with material previously expelled.
According to the Italian group, the scenario between the collision and the shell having a hole can be distinguished. After a collision, the shock wave first decreases but then it accelerates. This is exactly what has been found by Vink in the observations.
"When we recently found an acceleration and they predicted just that in their models, the puzzle pieces fell into place," said Vink.
Cassiopea A is located 11,000 light-years away in the eponymous constellation. The light of the supernova would have first arrived to Earth in 1670, but between intervening dust and limited telescope capabilities, nobody back then was able to see it.
Since then we have caught up with multiple observations, and now it is a favorite target for many observatories. NASA’s latest X-ray observatory IXPE released its first image last month and chose the supernova remnant as its subject. Excitingly, the JWST will study it in infrared later this year.