Astronomers still can’t decipher the ‘Cow,’ a mysterious explosion in deep space
|sciencemag.org 11 Jan 2019 at 08:40|
SEATTLE, WASHINGTONAn unusually bright glow in the sky that appeared suddenly last June has got astronomers in a frenzy. After months of study, they still arent sure what the objectofficially called AT2018cow, but universally referred to as theCowis. But scientists have some ideas, which they offered here today at the American Astronomical Society meeting. Whatever it is, says astronomer Liliana Rivera Sandoval of Texas Tech University in Lubbock, Its super weird.
The Cow first appeared in telescope observations on 16 June 2018, in what turned out to be a small galaxy about 200 million light-years away. It was very bright and hadnt been there the day before. That rapid appearance seemed to rule out a supernovabecause such stellar explosions usually grow in brightness more slowly. When we saw that we thought, lets get on this, says Daniel Perley, an astronomer at Liverpool John Moores University in the United Kingdom.
Astronomers initially assumed that the Cow was a much more nearby event, likely in our galaxy, and less cataclysmic than a supernova. One possibility was a white dwarfthe burnt-out remnant of a starconsuming material from a companion star, and sporadically flaring up in the process. Such events are common in the Milky Way. But analysis of AT2018cows light spectrum soon showed that the object was too far away, in another galaxya flaring white dwarf would never be visible at that distance.
Perley is one of the leaders of a global network of fast-reacting telescopes called GROWTH, and several of its instruments soon zoomed in on the Cow. These included the Liverpool Telescope on La Palma in Spains Canary Islandsand the Palomar Observatory in California. We dropped everything in the first 2 weeks, observing it seven times a night, he says.
The early observations confirmed the Cow was truly strange. It didnt show the telltale changes in its light output that a supernova would make, and it continued to grow in brightness and stayed bright and hot for nearly 3 weeks. These are things supernovae dont usually do, Perley says.
Its super weird.
Sandoval says as soon as she and colleagues knew AT2018cow was truly distant, they requested time on NASAs Neil Gehrels Swift Observatory to see what the Cow was doing in ultraviolet light and x-rays. The observations from the orbiting spacecraft revealed that the object was very bright in both those parts of the spectrum. Although the x-ray brightness fluctuated over the early weeks, the spectrum didnt change, there was no evolution there, which is very unusual, she notes. After 3 weeks, the x-ray signal began to fluctuatemore wildly while also dropping off in brightness.
Many astronomers agree that the long and steady duration of the event means that it was powered after an initial blast by some form of central engine. But what that engine may be is also far from clear. Some argue that it could be a very unusual supernova whose core has collapsed inward after the star exploded. Others say it is a tidal disruption eventa star being ripped apart by a black hole. But that usually requires the supermassive black hole in the center of a galaxy, and the Cow is situated in its galaxys spiral arm. So, some say, it could be a tidal disruption event spawned by an intermediate mass black hole , although evidence for the existence of such smaller black holes remains controversial. All explanations have problems, Sandoval says.
Four days after the Cows discovery, Anna Ho of the California Institute of Technology in Pasadena jumped into action with the Submillimeter Array on Mauna Kea inHawaii. Millimeter waves, at the short end of the radio spectrum, arent usually used to observe such exploding objects because the signal tends to die off so fast that telescopes cant catch it. The Cow was different. After several days it was still bright, Ho says. This is the first time weve ever seen [such a source] while it is brightening.
Just as at other wavelengths, the Cows submillimeter signal remained high and steady for several weeks, then began to tail off. Ho believes this signal shows the shock wave from whatever it was that originally exploded hitting a dense, surrounding cloud of gas and dust. When that happens, the cloud heats up and the gases emit light at various wavelengths. In this case, the emission continued as the shock wave traveled outwardthrough the cloud. The sudden drop-off in the submillimeter signal with time likely marked the shock reaching the outer limits of the gas cloud.
If astronomers find other such sources in the future, she says, studying the shock wave in this way would give them valuable data about thesize, velocity, and total energy of the shock, as well as the structure of the environment around the star. This tells us about what the star was doing before the explosion, says astronomer Bob Kirshner of the Gordon & Betty Moore Foundation in Palo Alto, California.