A Violent Cosmic Explosion Just Revealed the Birth of a Black Hole
The year 2018 might feel like decades ago, but when astronomers witnessed a violent explosion in a galaxy 200 million light-years away, it left everyone baffled. It was different than every other observed supernova — too forceful and blindingly bright. Initially called AT2018cow, the scientific community soon gave it a more relatable name: the Cow.
In the intervening years, the transient (or short-lived) event remained a mystery — it might have been a star ripped into pieces by a nearby black hole, or it could've been a "failed supernova" event — where a black hole consumes a star from the inside out. But to confirm which scenario was true, astronomers needed another Cow-like explosion.
And, recently, they got what they wanted.
Starting on Oct. 12, 2020, telescopes captured an event in a galaxy 3 billion light-years away that grew overwhelmingly bright, but then disappeared forever. This rapid series of events almost exactly mirrors the transient Cow, which is why astronomers posted a paper to a preprint server — where they gave the second cow a more illustrative name: the Camel.
Astronomers prepared to capture vital data on black hole birth
"It's really exciting," said Astrophysicist Deanne Coppejans of Northwestern University, in a Quanta Magazine report. "The discovery of a new transient like AT2018cow shows that it's not a complete oddball. This is a new type of transient that we're looking at."
For astronomers, the Cow event was unprecedented — lacking any obvious explanation. But Camel came after the scientific apparatus had unfolded to capture exactly what was happening. "We were able to realize what it was within a few days of it going off," said Astrophysicist Daniel Perley of Liverpool John Moores University — who also led the study — to Quanta.
"And we got lots of follow-up data," added Perley.
Days later, the research team trained telescopes in Hawaii and the Canary Islands to gather crucial data and characterize the exceedingly violent cosmic explosion. They even raised a worldwide alarm for other astronomers to look — on a service called "Astronomer's Telegram."
Similar 'zombie afterlife behavior' of massive star deaths
The new "Camel" event had two names at first: the AT2020xnd, which came from a global catalog of all transient events — and another, called ZTF20acigmel — which came from the Zwicky Transient Facility that initially spotted the explosion in space. The latter name was morphed into something more relatable — "Camel" — "Xnd didn't quite have the same ring to it," said Perley, to Quanta.
Much like the Cow, the Camel burned extremely bright in a short time, achieving its peak brightness in two to three days. It became roughly 100 times brighter than typical supernovae, and then dimmed in days instead of weeks. "It fades very fast, and while it's fading it stays hot," Perley said.
There were two additional Cow-like events astronomers studied before the Camel — called CSS161010 and "Koala" — and while they fit the story of getting bright and fading fast, and looking blue from the immense heat generated, these "fast blue optical transients" are not the same.
"The explosion itself and the sort of zombie afterlife behavior, those are quite similar," said Astrophysicist Anna Ho of the University of California, Berkeley — who first discovered Koala and was later involved in the Camel discovery team. The sequence seemed like an explosion from a star colliding with nearby dust and gas.
Black holes like the Camel and Cow fire powerful jets through the outer layers of dead stars
However, "the collision stage where you're seeing the explosion collide with ambient material, that has shown some variation in the amount of material lying around and the speed in which the shock wave from the explosion is plowing through the material."
In other words, what the scientists witnessed was likely something predicted by the failed-supernova hypothesis — where a star roughly 20 times our sun's mass exhausts its internal fuel and begins to die. As the core collapses, the infalling material of stars this size typically "bounces" outward — leaving only a dense object behind: a neutron star.
The Camel and Cow appear to break with this norm, and "something unusual happens in the process to core collapse," Perley explained to Quanta. "What we claim is that instead of collapsing to a neutron star, it collapsed straight into a black hole, and most of the star fell into the black hole."
Jarringly, as the black hole consumes the outer layers of a dead star, it starts to spin, faster and faster, until it generates strong and deadly jets that shoot out from the poles — creating the explosion of light we witnessed as the jets force their way through the outer layers of the star.
Not all black holes are born equal — not only in size, but in the way they emerge into existence. While they're one of the most fundamentally violent forces known to exist since the big bang, black holes also create wildly strong centers of gravity for some galaxies (namely, ours). But since their internal processes are practically impossible to study from outside the event horizon — beyond which not even light can escape — further analyses of births like the Camel and Cow could help us understand these monsters haunting the depths of outer space.