New type of space explosion reveals birth of black hole
In 2018, astronomers were shocked to find a bizarre explosion in a galaxy 200 million light years away. It wasn’t like any normal supernova seen before – it was both shorter and brighter. The event received an official designation, AT2018cow, but it quickly went by a more jovial nickname: the cow.
The fleeting event – known as the transient – defied explanation. Some thought it might have been a star torn apart by a nearby black hole, but others preferred a “failed supernova” scenario, where a black hole literally eats a star from within. To find out for sure, they needed to find more cow-like events.
Over two years later, they had one.
As of October 12, 2020, telescopes saw something in a galaxy 3 billion light years away become incredibly bright and then disappear from view. It behaved almost the same as the cow, reported astronomers in a document posted on the arXiv.org online pre-print site last week, leading them to conclude that it must be the same type of episode. In keeping with tradition, it received its own animal-inspired name: the camel.
“It’s really exciting,” said Deanne Coppejans, astrophysicist at Northwestern University. “The discovery of a new transient like AT2018cow shows that it’s not completely weird. This is a new type of transient that we are looking at. “
The cow was a total surprise, and astronomers weren’t quite sure what they were looking at when she appeared. The camel, on the other hand, was like a burglar setting off the new alarm system. “We were able to achieve what it was in a matter of days,” said Daniel Perley, an astrophysicist at John Moores University in Liverpool who led the new study. “And we have a lot of tracking data.”
Four days later, the team used telescopes in the Canary Islands and Hawaii to obtain vital data on its properties. They later issue an alert to other astronomers on a service called the Astronomer’s Telegram.
The event received two designations. One, AT2020xnd, was from a global catalog of all transients, and the other, ZTF20acigmel, was from the Zwicky Transient Facility, the telescope where it was discovered. The team twisted the latter into his nickname “camel”. “Xnd didn’t have quite the same ring,” Perley said.
Like its predecessor, the Camel got very shiny in a short period of time, reaching its peak in two or three days. It has become about 100 times brighter than any normal type of supernova. Then it quickly subsided in a process that only lasted a few days, rather than weeks. “It fades really fast, and while it fades, it stays hot,” Perley said.
Prior to this discovery, astronomers had sifted through historical data to find two additional cow-like events, the “Koala” and CSS161010, but the camel is the first to be seen in real time and therefore studied in detail from the cow.
All four events have similar properties. They quickly turn shiny, then quickly fade. They are also hot, which makes them blue. But these “fast blue optical transients” are not the same.
“The explosion itself and the kind of behavior of zombies in the afterlife are quite similar,” said Anna Ho, an astrophysicist at the University of California at Berkeley, who discovered the Koala and was part of the Camel Discovery Team. The events all appear to be some sort of explosion of a star colliding with gas and dust nearby. “But the collision stage where you see the blast colliding with the surrounding material, which showed some variation in the amount of material lying around and the speed at which the blast’s shock wave passes through the material.”
The main idea at the moment is failed supernova hypothesis. The process begins when a massive star about 20 times the mass of our sun reaches the end of its life and runs out of fuel. Its core then collapses, starting what would normally be a regular supernova, where the infallible material bounces off, leaving behind a dense object called a neutron star.
But in cases like the camel and the cow, “something unusual is happening in the process of heart collapse,” Perley said. “What we are claiming is that instead of collapsing into a neutron star, it collapsed directly into a black hole, and most of the star fell into the black hole.”