Astronomers debate an unprecedented “holy cow” outburst

X marks the spot: the white lines mark the location of an outburst known as “the Cow” in a galaxy 200 million light years away. Astronomers argue the explosion of light could be due to a supernova giving birth to a neutron star or black hole. Others say the data shows it more likely was the result of a massive black hole tearing a passing white dwarf apart in a tidal disruption event. Image: Sloan Digital Sky Survey

On 16 June 2018, astronomers spotted a celestial outburst in a galaxy 200 million light years away that was unlike any ever seen before. Over three days, the object known as AT2018cow – or “the Cow” for short — emitted a torrent of radiation 10 times brighter than a typical supernova that slowly faded away over several months.

The outburst occurred in a galaxy known as CGCG 137-068 in the constellation Hercules and was first observed with the Asteroid-Terrestrial-impact Last Alert System – ATLAS – telescope in Hawaii.

Using data from multiple instruments around the world and in space, including the Neils Gehrels Swift Observatory and the Nuclear Spectroscopic Telescope Array, or NuSTAR, two groups of astronomers have come up with two possible explanations.

The left image shows the galaxy CGCG 137-068 before the Cow was observed, with a green circle marking its future location. The Liverpool Telescope in the Canary Islands captured the Cow close to peak brightness on 20 June 2018 (center). The right frame shows the view from the William Herschel Telescope about a month after the Cow reached maximum brightness. Image: Daniel Perley, Liverpool John Moores University

In one, a white dwarf was torn apart in a cataclysmic tidal disruption event when it passed too close to a black hole with a mass ranging from 100,000 to 1 million times that of the sun. The doomed dwarf was ripped asunder by the black hole’s enormous gravity, breaking apart in a stream of gas that whipped back around the hole and collided with itself, creating a huge elliptical cloud of debris.

“We’ve never seen anything exactly like the Cow, which is very exciting,” said Amy Lien, an assistant research scientist at t 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 multi-wavelength observations as it faded over the next few months.”

Lien is co-author of a paper describing the observations in an upcoming edition of Monthly Notices of the Royal Astronomical Society. The video below shows a visualisation of how the tidal disruption event might have proceeded.

But a different group of researchers argues the Cow outburst was the result of a massive star exploding in a supernova, giving birth to a collapsed neutron star or black hole. If so, it would mark the first time such an event has been observed as it happened.

“We saw features in the Cow that we have never seen before in a transient, or rapidly changing, object,” said Raffaella Margutti, an astrophysicist at Northwestern University in Illinois and lead author of a study about the Cow to be published in The Astrophysical Journal.

“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.”

Data from NASA’s NuSTAR and the European Space Agency’s XMM-Newton and INTEGRAL satellites, along with the Very Large Array, indicate the optical and ultraviolet flash initially detected was generated by a supernova blast and that X-ray emissions seen after the outburst came from gas heating up as it fell toward a neutron star or black hole.

“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 co-author 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.”