Almost four years ago, astronomers discovered a supernova that didn’t fit in: AT2018cow – simply known as the "Cow" – was brighter and shorter than other such events. It turned out to be the first of several unusual stellar explosions. Now, the brightest “cow” in X-ray has been observed.
The findings were presented virtually at the 239th meeting of the American Astronomical Society by Yuhan Yao, from Caltech. Yao and colleagues reported on the event known as AT2020mrf, first discovered in X-rays thanks to the Russian--German Spektrum-Roentgen-Gamma (SRG) telescope.
The new event appeared 20 times brighter than the original Cow, and it kept growing. One year later, observations conducted by NASA’s Chandra X-ray Observatory showed that AT2020mrf was 200 times brighter than the cow after the same time interval.
"When I saw the Chandra data, I didn't believe the analysis at first," Yao said in a press statement seen by IFLScience. "I reran the analysis several times. This is the brightest Cow supernova seen to date in X-rays. We can see down into the heart of these explosions to directly witness the birth of black holes and neutron stars.”
That is likely what’s powering the peculiar explosions. The Cow events are unobscured explosions where we can see the formation of an extremely compact object; either a stellar-sized black hole or a neutron star. They are very active, releasing their power through X-ray emission.
"The large amount of energy release and the fast X-ray variability seen in AT2020mrf provide strong evidence that the nature of the central engine is either a very active black hole or a rapidly spinning neutron star called a magnetar," Yao continued. "In Cow-like events, we still don't know why the central engine is so active, but it probably has something to do with the type of the progenitor star being different from normal explosions."
To understand why they are different, it is important to discover more examples. Only a handful are known, and given how different and powerful AT2020mrf is, it is likely that this class of stellar explosions has more variety than previously expected.
"Finding more members of this class will help us narrow in on the source of their power," Yao added.
The work is submitted for publication in The Astrophysical Journal and it is available on the paper repository ArXiv.