The sudden appearance of a powerful blast from a nearby galaxy last year sent astronomers around the world into a spin.
It soon became clear the explosion, called AT2018cow — or simply “the Cow” — was like nothing the astronomers had seen before.
After chasing down the Cow for more than six months, they are still baffled.
But a lot of the latest data, presented today at the 233rd meeting of the American Astronomical Society in Seattle, points to an entirely new type of supernova created by a massive star that collapsed to form a black hole or a fast-spinning neutron star known as a magnetar.
If so, it would be the first time we have ever seen such an event in real time, said Raffaella Margutti of Northwestern University in Chicago, who is presenting her team’s work at the conference.
Other astronomers say there is still an outside chance the explosion could have been caused by a star being ripped apart by a black hole — a phenomenon known as a tidal disruption event.
Whatever it is, it does not fit any of the models, said Daniel Perley, an astronomer from Liverpool John Moores University in the UK who led another of the international teams.
“It has very different properties from anything we’ve seen before,” Dr Perley said.
Not your regular supernova
The Cow lies in a dwarf galaxy 200 million light years away from us in the constellation of Hercules.
When word got out that a strangely bright object had been spotted in our cosmic neighbourhood by the ATLAS telescopes on June 16, astronomers scrambled to observe its rise and fall using a global network of telescopes.
“It may well be the most intensely observed astronomical source ever,” Dr Perley said.
“The only thing that competes would be the gravitational wave detection from a year-and-a-half ago,” he said, referring to the observation of fireworks created by the collision of two ancient stars.
Dr Perley and his team used optical telescopes to follow the Cow every night for more than a month.
While supernovae take two to three weeks to reach their peak, the Cow became extremely bright within two to three days before quickly fading. At its most brilliant, it was the equivalent of 100 billion times the luminosity of our Sun — much brighter than a typical supernova.
An even more intense picture of the Cow started to emerge as other astronomy teams came on board to observe the event in other parts of the energy spectrum, from radio waves to X-rays, as the drama unfolded over the next six months.
Tara Murphy and her PhD student Dougal Dobie of the University of Sydney used the CSIRO’s Australia Telescope Compact Array at Narrabri in New South Wales to observe radio emissions created by the blast’s shockwave.
“What we can see is that the shockwaves were travelling about a 10th the speed of light and we saw the brightness of this source in radio keep increasing over time,” Dr Murphy said.
The increasing brightness was also observed in shorter wavelengths by team leader and Caltech astronomer Anna Ho and her colleagues, using telescopes in Hawaii and Chile.
“The fact it was getting brighter with time and particularly in higher frequencies shows there must be something that was still powering the explosion pumping energy into this material,” Dr Murphy said.
“It wasn’t just this explosion that happened and then was fading away, there must be something that was still there that makes it different from the typical supernova we see.”
She said the radio data pointed to the presence of a magnetar at the core of the supernova.
Central engine at the heart of the Cow
Dr Margutti’s team were also following the Cow, using 16 telescopes around the world to study radio waves, gamma rays, and a spectrum not usually used to study supernovae: hard X-rays, which are 10 times more powerful than normal X-rays.
Unlike a regular supernova, the Cow has very little material swirling around it, which it enabled the team to peer straight into its heart.
“The most striking feature in the Cow is what we find in the hard X-rays,” Dr Margutti said.
These powerful X-rays indicated that lying deep within the core of the Cow was a central engine that continued to pump out energy for at least 100 days after the blast.
This kind of long-lasting energy source is not known in normal supernovae or gamma rays, but could occur as material swirls around a compact object such as a newly forming black hole or fast-spinning magnetar.
“The big question we’re trying to find out is why in the first place we have a little amount of mass.”
Dr Margutti said the blast could have been created by the death of a massive blue supergiant star.
“There is a possibility that those types of stars just go directly into a collapse without producing even a little bit of an explosion.”
Dr Margutti said the radio data indicated the Cow was not created by a pre-existing black hole stripping a star.
Black holes thought to be big enough to strip a star on the outskirts of a galaxy, where the Cow was found, are believed to form when stars merge in low-density environments such as globular clusters.
Instead, she said, the Cow formed in a high-density environment more consistent with those around massive stars, which enrich their environment with their winds before they collapse.
Dr Perley said the exotic supernova hypothesis was the most compelling to date, but it did not explain all the features seen in the optical data, such as why the central engine was so opaque.
“They’ve done a very good job of explaining it as well as you can with the tools we have right now,” Dr Perley said.
He also agreed there were a number of problems with the alternative hypothesis of a black hole ripping a star apart, but said it was too early to rule out yet.
“Previous partial disruptions of the star could have produced some pre-existing dense gas, or the black hole could have an accretion disk,” he explained.
A new type of cosmic bang
Brad Tucker, of the Australian National University, did not study the Cow but has investigated supernova events detected by Kepler and now, the TESS telescope.
Of all the hypotheses put forward by the teams, he leans towards the creation of a black hole by a failed supernova.
“I don’t know if the data favours this one way or the other. But the reason I like that is it’s simple — it doesn’t require any special physical mechanism.
“We know blue supergiants exist and explode. We know they explode at different regimes. What’s not to say that this could create a black hole?
“Every time we see a blue supergiant, something is weird about it.”
Less likely, he thought, was the idea of a tidal disruption event.
“If it’s not a supernova we always say, ‘Is this a tidal disruption event?’ and you take a look at it and generally the answer is ‘no’,” Dr Tucker said.
“But again, these are things we’re just finding more and more of and just trying to understand.
“We’re narrowing down and ruling things out, but what we can definitively say is there is another type of explosion or type of star that goes bang in the night that we’re just starting to uncover.”
The focus now is to find more objects like the Cow to definitely work out how they were formed, Dr Perley said.
“This shouldn’t be one of a kind,” he said.
“There were some events that had similar properties that were seen in previous surveys, but no-one really recognised them … and they were never followed up in real time or even discovered in real time.
“So, now we know what to look for, we’re pushing forward to find more and to get more data.”