Something strange happened in the sky one summer night in 2018, and astronomers are still trying to figure out exactly what occurred. But their telescopes were in the right position to capture a mysterious bright object flaring in the sky before it vanished.
Officially, it’s AT2018cow, a computer-generated catalog name. But researchers much prefer its unofficial name: “The Cow.”
In mid-June, a cosmic flare was registered by astronomers using the ATLAS asteroid survey, or Asteroid Terrestrial-impact Last Alert System. The flare grew into a sudden burst of bright light from the direction of the dwarf spiral galaxy CGCG 137-068, 200 million light-years away in the Hercules constellation. In terms of galactic neighbors, that’s relatively close to us, according to astronomers.
Then, the bright anomaly vanished as quickly as it arrived.
The ATLAS astronomers immediately alerted the rest of their community.
It captured the attention of astronomers around the globe and was observed using multiple telescopes registering data in optical light, radio wavelengths, X-rays, gamma rays and infrared. They continued to observe it long after the initial brightness faded.
The astronomers realized that the fast, bright blue event was no normal supernova, an increasingly bright star that explodes and ejects most of its mass before dying.
“This was an incredibly luminous event, brighter than almost any supernova we’ve ever seen before,” said Daniel Perley, assistant professor of astronomy at Liverpool John Moores University. “The Cow also appeared and faded away very quickly: so quickly that existing supernova models can’t properly explain it. It must be a new type of extremely energetic, explosive event.”
Perley is one of several astronomers who presented their findings on the Cow at the 223rd meeting of the American Astronomical Society on Thursday.
In fact, the anomaly was 10 to 100 times brighter than the average supernova, and it flared and disappeared quicker than other star explosions. The particles within the explosion were traveling at 30,000 kilometers per second – 10% of the speed of light – and the activity died down after about 16 days.
The SOAR telescope in Chile offered a closer look that allowed astronomers to determine that hydrogen and helium were present. If this had been a merger of objects, like those that produce gravitational waves, heavy metals would have been present instead.
Because the Cow was more “naked” than other explosions, with 10 times less particles swirling around it, astronomers were able to look into the center, revealing a “central engine.”
“A ‘lightbulb’ was sitting deep inside the ejecta of the explosion,” said Raffaella Margutti, an astrophysicist at Northwestern University, in a statement. “It would have been hard to see this in a normal stellar explosion. But the Cow had very little ejecta mass, which allowed us to view the central engine’s radiation directly.”
Rather than fading steadily, like most supernovae, there were bumps and wiggles in the data, suggesting that something was powering the material as it expanded out. The light seemed to spike every few days.
Other astronomers arrived at a similar conclusion, believing that it was an “engine-driven” explosion powered by a black hole or magnetar, a fast-spinning neutron star that forms in a supernova.
Multimessenger astronomy, a combination of observations and data across the electromagnetic spectrum, helped astronomers come up with several scenarios for that this could be.
“We think that ‘The Cow’ is the formation of an accreting black hole or neutron star,” Giacomo Terreran, observation lead for the Cerro Tololo Inter-American Observatory, said in a statement. “We know from theory that black holes and neutron stars form when a star dies, but we’ve never seen them right after they are born.”
If that’s the case, the telescopes captured the exact moment when a star collapsed, forming a black hole or a neutron star. The bright glow was due to debris swirling around the object. And observing this event with so many imaging sources could give astronomers a better understanding of the physics occurring in the first crucial moments that go into creating a neutron star or black hole.
Astronomers continue to debate what exactly happened. Liliana Rivera Sandoval, a postdoctoral research fellow in the Texas Tech University Department of Physics and astronomy, believes that it was an uncommon core collapse supernova: the rapid collapse of a massive star that violently exploded. Sandoval and her colleagues used the Swift Observatory to obtain some of the first measurements of the event.
There are other possibilities.
Was the Cow a supernova that left behind a magnetar in its wake?
“If we’re seeing the birth of a compact object in real time, this could be the start of a new chapter in our understanding of stellar evolution,” said Brian Grefenstette, a NuSTAR instrument scientist at Caltech. “We looked at this object with many different observatories, and of course the more windows you open onto an object, the more you can learn about it. But, as we’re seeing with the Cow, that doesn’t necessarily mean the solution will be simple.”
Or did a black hole rip apart a white dwarf star?
“The Cow produced a large cloud of debris in a very short time,” said Paul Kuin, an astrophysicist at University College London. “Shredding a bigger star to produce a cloud like this would take a bigger black hole, result in a slower brightness increase and take longer for the debris to be consumed.”
It could also be a new class of object, a fast luminous transient, known as a weak supernova that cools quickly. New telescopes will be able to help spot more of these events in the future.
“The properties of the Cow strain nearly all models we have tried to devise to explain it,” Perley said. “Whatever it is, it must involve some form of energetic and very fast explosion interacting with an extremely dense shell of material very close to the explosion progenitor.”