SEATTLE, WASHINGTON—An unusually bright glow in the sky that appeared suddenly last June has got astronomers in a frenzy. After months of study, they still aren’t sure what the object—officially called AT2018cow, but universally referred to as the “Cow”—is. 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, “It’s 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 hadn’t been there the day before. That rapid appearance seemed to rule out a supernova because such stellar explosions usually grow in brightness more slowly. “When we saw that we thought, let’s 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 dwarf—the burnt-out remnant of a star—consuming material from a companion star, and sporadically flaring up in the process. Such events are common in the Milky Way. But analysis of AT2018cow’s light spectrum soon showed that the object was too far away, in another galaxy—a 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 Spain’s Canary Islands and 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 didn’t 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 don’t usually do,” Perley says.
It’s super weird.
Sandoval says as soon as she and colleagues knew AT2018cow was truly distant, they requested time on NASA’s 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 didn’t change, there was no evolution there, which is very unusual,” she notes. After 3 weeks, the x-ray signal began to fluctuate more 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 event—a 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 galaxy’s 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 Cow’s discovery, Anna Ho of the California Institute of Technology in Pasadena jumped into action with the Submillimeter Array on Mauna Kea in Hawaii. Millimeter waves, at the short end of the radio spectrum, aren’t usually used to observe such exploding objects because the signal tends to die off so fast that telescopes can’t catch it. The Cow was different. “After several days it was still bright,” Ho says. “This is the first time we’ve ever seen [such a source] while it is brightening.”
Just as at other wavelengths, the Cow’s 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 outward through 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 the size, 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.
As so often happens, what researchers need is more data. “I hope there are more Cows,” Sandoval says.