On June 17th 2018, the ATLAS (Asteroid Terrestrial-impact Last Alert System) survey’s twin telescopes spotted something extraordinarily bright in the sky. The source was 200 million light years away in the constellation Hercules. The object was given the name AT2018cow or “The Cow.” The Cow flared up quickly, and then just as quickly it was gone.
What was it?
At first, the obvious answer was a supernova. When a transient bright spot flares up in the sky, that’s the first thing astronomers think, and the first thing they try to confirm, or rule out.
There are different types of supernovae. The most well-known occurs at the end of a massive star’s life. In this case, a star can no longer sustain fusion in its core. Eventually, gravity overpowers the star and it collapses. This causes a massive explosion that outputs an enormous amount of energy for several months.
Another kind is caused by a white dwarf in a binary system with another star. A white dwarf can accrete matter from its companion until it reaches a critical mass and runaway fusion happens. This type of supernova can be 5 billion times brighter than the Sun.
Raffaella Margutti, Northwestern University
“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. Never.”
Supernova aren’t exactly rare, depending on how you look at it. There are about three of them every hundred years in the Milky Way. But with all the powerful telescopes on Earth, we see them well outside our own galaxy because they’re so bright. ATLAS, the telescope system that spotted the Cow, has spotted 2,795 supernovae since it began operations in 2015. Amateur astronomers even discover them.
But all supernovae light up for several months. The Cow didn’t last that long. It only flared brightly for about 16 days. Its short duration ruled out a supernova.
“We knew right away that this source went from inactive to peak luminosity within just a few days,” Margutti said. “That was enough to get everybody excited because it was so unusual and, by astronomical standards, it was very close by.”
Like a supernova, the Cow was also extraordinarily bright. But it was between 10 to 100 times brighter than a supernova. For astronomers, watching an active supernova is exciting, but the Cow was something else, and nobody was sure what. Excitement and curiosity spread.
“We thought it must be a supernova,” Margutti said. “But what we observed challenged our current notions of stellar death.”
Raffaella Margutti, Northwestern University
“We think that ‘The Cow’ is the formation of an accreting black hole or neutron star.”
A team of astronomers led by Raffaella Margutti at Northwestern University went to work. Astronomers study stellar deaths in visible light, but the team combined several types of observations from different telescopes to help them understand what they were seeing. They analyzed the Cow’s chemical composition, and it included hydrogen and helium. That ruled out merging compact objects as a cause.
They gathered images in hard X-rays, X-rays, gamma waves, and radio-waves. They used a who’s who of telescopes to study the Cow, including the Keck Observatory, the MMT Observatory, the SoAR telescope, NuSTAR, INTEGRAL, and XMM Newton to study the Cow. Since they weren’t relying only on visible light, they were able to study the Cow long after its initial flaring had died down. They began to speculate on what they were seeing.
“Our team used high-energy X-ray data to show that the Cow has characteristics similar to a compact body like a black hole or neutron star consuming material. But based on what we saw in other wavelengths, we think this was a special case and that we may have observed – for the first time – the creation of a compact body in real time.”
The team thinks they’ve witnessed the exact moment that a supernova collapsed to form a compact object; either a black hole or a neutron star.
“We think that ‘The Cow’ is the formation of an accreting black hole or neutron star,” said Northwestern’s Raffaella Margutti, who led the research. “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. Never.”
Stars collapse into black holes and neutron stars all the time. The event itself isn’t exactly rare. But seeing it happen is rare. Margutti and the rest of the team were able to study the Cow so closely because in a way, it was naked.
Typically, when a star collapses into a black hole or a neutron star, the event is hidden from sight inside a shroud of gas and dust left over from the preceding supernova. In this case, there was 10 times less material than their normally is. The astronomers were able to look directly into the heart of the event, called the “central engine.”
“A ‘lightbulb’ was sitting deep inside the ejecta of the explosion,” Margutti said. “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.”
The Cow is also close to us, in astronomical terms, making it easier to study. It’s only 200 million light years away, in the dwarf galaxy CGCG 137-068.
Margutti and her team presented their results at the 233rd American Astronomical Meeting last week. Their results are also published in a paper.
Another team of astronomers, led by Paul Kuin from, an astrophysicist at University College London, came to a different conclusion. They think that the Cow is a star that’s been ripped apart in what’s called a “tidal disruption event.”
Amy Lien, University of Maryland, NASA’s Goddard Space Flight Center
“We’ve never seen anything exactly like the Cow, which is very exciting.”
A tidal disruption event happens to a star that approaches a powerful black hole. The gravity from the black hole tears the star apart into a stream of gas. The black hole flings the tail of the gas stream out of the black hole system, but the leading edge of the stream swirls around the hole and eventually collides with itself.
“We’ve never seen anything exactly like the Cow, which is very exciting,” said Amy Lien, an assistant research scientist at the University of Maryland, Baltimore County and NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “We think a tidal disruption created the quick, really unusual burst of light at the beginning of the event and best explains Swift’s multiwavelength observations as it faded over the next few months.”
This second team of researchers think that the star that got shredded was a white dwarf, the same type of star that our Sun will eventually become. They also calculated the size of the black hole and concluded that it’s mass would be between 100,000 to 1 million times the Sun’s. This makes it an enormous black hole, similar in size to the one at the center of the Cow’s galaxy. That would be very unusual, but not impossible.
“The Cow produced a large cloud of debris in a very short time,” said lead author Paul Kuin, an astrophysicist at University College London (UCL). “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.”
Kuin’s team also shared their findings at the 233rd AAS Meeting, and their results were published in a paper.
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