A short and unusual flash discovered in the night sky on June 16, 2018, puzzled astronomers and astrophysicists all over the world. The event called AT2018cow and the nick name "The Cow" after the occasional final letters in its official name – unlike any heavenly outbreak ever seen, which led to several theories of its source.
For three days, the cow produced a sudden explosion of light at least 10 times brighter than a typical supernova, and then faded in the next few months. This unusual event occurred in or near a star-forming galaxy called CGCG 137-068, located approximately 200 million light-years away in the Hercules constellation. The cow was first observed by the NASA-funded Asteroid Terrestrial Impact Load Alert System telescope in Hawaii.
So exactly what is the cow? Using data from several NASA missions, including Neil Gehrels Swift Observatory and Nuclear Spectroscopic Telescope Array (NuSTAR), two groups publish papers that provide possible explanations for the origin of the cow. One paper claims that the cow is a monster black hole that shreds a suitable star. The second document anticipates that it is a supernova-stellar explosion that gave birth to a black hole or a neutron star.
Researchers from both teams shared their interpretations at a panel discussion on Thursday, January 10, at the 233rd American Astronomical Society meeting in Seattle.
And Black Hole Shredding a Compact Star?
A potential explanation of the cow is that a star has been torn together in which astronomers call a tidal interruption event. Just as the Moon's gravity causes the Earth's sea to bump, create tides, a black hole has a similar but more powerful effect on a nearing star, and ultimately breaks it into a stream of gas. The tail of the gas flow is knocked out of the system, but the leading edge swings back around the black hole, colliding with itself and creating an elliptical cloud of material. According to a research group using data ranging from infrared radiation to gamma rays from Swift and other observatories, this transformation explains best cow behavior.
"We have never seen anything like the cow, which is very exciting," said Amy Lien, an assistant researcher at the University of Maryland, Baltimore County and NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We believe that a tidal break created the rapid, tremendous 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 and her colleagues find that the shredded star was a white dwarf – a warm, coarse ground size that marks the final state of stars as our Sun. They also calculated that the black hole mass varies from 100,000 to 1 million times the sun, almost as large as the central black hole in its host galaxy. It is unusual to see black holes on this scale outside the center of the galaxy, but it is possible that the cow occurred in a nearby satellite galaxy or a global star cluster whose older star populations could have a higher proportion of white dwarfs than average galaxies.  Holy cow! Mystical blast was studied with NASA telescope “/>
A paper describing the findings, co-authored by Lien, will be displayed in a future edition of the journal Monthly Messages from the Royal Astronomical Society .
"The cow produced a large cloud of debris in a very short time," says lead author Paul Kuin, an astrophysicist at University College London (UCL). "Shredding a larger star to produce a cloud like this would take a bigger black hole, results in a slower brightness and take longer for debris to be consumed."
Or a new view of a supernova?
Another team of researchers was able to collect data about the cow over an even wider wavelength range, which ranges from radio waves to gamma rays. Based on these observations, the team suggests that a supernova could be the source of the cow. When a massive star dies, it explodes as a supernova and leaves behind either a black hole or an incredibly dense object called a neutron star. The cow could represent the birth of one of these star remains.
"We saw features in the cow that we have never seen before in a transient or fast-changing object," said Raffaella Margutti, an astrophysicist at Northwestern University in Evanston, Illinois, and leads an author's study of the cow to be published in Astrophysical Journal . "Our team used highly efficient X-ray data to show that the cow has properties 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 might have observed – for the first time – creation of a compact body in real time. "
Margutt's team analyzed data from several observatories, including NASA's NuSTAR, ESA (European Space Agency) XMM-Newton and INTEGRAL satellites, and the National Science Foundation's Very Large Array. The team suggests that the light optical and ultraviolet flash from the cow signaled a supernova and that the X-ray emissions that followed shortly after the outbreak arose from gas that radiated energy when it fell on a compact object.
Usually, a supernova's expanding debris cloud blocks some light from the compact object in the center of the explosion. Due to the X-ray emissions, Margutti and her colleagues suggest that the original star in this scenario may have been relatively low in mass, giving a relatively thinner junk cloud through which X-ray from the central source could escape.
"If we see the birth of a compact object in real time. It could be the beginning of a new chapter in our understanding of star development," said Brian Grefenstette, a NuSTAR instrument researcher at Caltech and a co-author of Margott's paper. "We looked at this object with many different observatories, and the more windows you open on an object, the more you can learn about it. But as we see with the cow it does not necessarily mean the solution will be simple."
Birth of a black hole or neutron star captured for the first time
Swift Spectra by AT2018cow: A White Dwarf Tidal Disruption Event? arxiv.org/abs/1808.08492