Images taken by the ATLAS telescopes before the explosion, middle, and after, left, show the sudden brightening in the galaxy CGCG 137-068. The image on the right shows the difference between the two. (Stephen Smartt/ATLAS)

When a brilliant, bewildering celestial body flashed into being in a nearby galaxy over the summer, it transfixed the entire astronomy community.

No one had ever witnessed anything like it. Over the course of just a few days, the cosmic flare grew until it was almost 100 billion times as bright as the sun. Soon after the sighting was reported, scientists were observing it in every wavelength of the electromagnetic spectrum.

Yet even after those hundreds of hours of effort, the signal defies easy explanation — posing the tantalizing possibility that astronomers have detected something entirely new.

In published studies and presentations Thursday at the winter meeting of the American Astronomical Society in Seattle, scientists debated what astrophysical phenomenon could be behind this puzzling flare.

Several research teams argue that it may come from the birth of a black hole or a collapsed star. Only an incredibly dense object shining through a cloud of stellar debris could power such a fast and brilliant signal, these scientists say.

Still others argue the flare was emitted by a star in its death throes as it was devoured by a black hole.

Events like this one “are new frontiers for discovery,” said Northwestern University astrophysicist Raffaella Margutti, who led research suggesting the flare comes from a newborn black hole. Whatever the origins of this signal, studying it — and finding more — could allow scientists to probe the universe’s most exotic locales where the laws of physics are pushed to their extremes.

AT2018cow erupted in or near a galaxy known as CGCG 137-068, which is located about 200 million light-years away from Earth in the constellation Hercules. The yellow cross shows the location of this puzzling outburst. (Sloan Digital Sky Survey)

The June 16, 2018, detection came from Hawaii’s ATLAS telescope, which surveys the night sky for supernovas and other powerful, short-lived phenomena.

These events, known as “transients,” typically take a few weeks to reach full brightness. But this flare unfolded even faster. By day three, it was about 10 times as bright as a standard exploding star, and it sent a shock wave zooming outward at 20,000 miles a second — 10 percent of the speed of light.

An urgent message was posted on the Astronomer’s Telegram, an online service for astronomers to rapidly report interesting observations. Thanks to the site’s randomized three-letter naming system, the object was dubbed AT2018cow, or “the Cow” for short.

And the world took notice. Within a week, the ATLAS observation was followed up by more than two dozen teams of astronomers using telescopes based on at least four continents and in space.

“There’s a lot of interest,” Robert Rutledge, editor in chief of the Astronomer’s Telegram and an astrophysicist at McGill University in Canada, said at the time. “I think it’s the most notices for any individual object in such a short period of time.”

Several studies suggested the object was a mere 200 million light-years away — practically in our backyard, by cosmic standards.

Yet the Cow seemed to strain every model devised to explain it, said Dan Perley, an astronomer at Liverpool John Moores University in the United Kingdom. It carried light signatures of hydrogen and helium — typical elements for a star like our own sun — rather than the carbon, oxygen and nitrogen usually seen in a supernova.

Caltech researcher Anna Ho led an investigation into the event using millimeter wavelengths, a part of the electromagnetic spectrum that is slightly higher in energy than radio waves, but which is rarely used to study supernova. To their surprise, the Cow kept getting brighter in millimeter wavelengths — something astronomers have never seen before.

“There wasn’t just a single release of energy that happened and then it was over. There had to be energy that was still being produced,” Ho said. “There had to be an active engine.”

Margutti was among the scores of scientists who sprang into action to seek that “engine.” Like Captain Planet uniting the Planeteers, she called on colleagues around the globe to study the Cow over the next 100 days. Each team member specialized in a different part of the electromagnetic spectrum and could interpret whatever insights about the signal were conveyed in that wavelength of light.

“It was a great adventure,” she recalled. “A lot of the time in research you sort of know what you’re looking at. But our team was facing a situation where we had tons and tons of data … and no clue how to explain it.”

Artist’s conception of a cosmic blast with a “central engine,” such as that suggested for AT2018cow. The black hole at center is pulling in material that forms a rapidly rotating disk that radiates prolific amounts of energy and propels super-fast jets of material from its poles. (Bill Saxton/NRAO/AUI/NSF)

The most intriguing clue came from a “bump” in the high-energy X-rays. This bump is an unusual sight around a supernova, but it’s a characteristic feature of an accretion disk — the halo of superheated material that swirls around an extremely dense object.

This signal looked like it could be coming from a black hole or a neutron star — the dark, dense nugget left behind when a midsize star collapses.

Though theories suggest these kinds of compact bodies are born in the aftermath of supernovas, scientists are never able to witness the initial stages of the formation process. The clouds of debris left behind after these cataclysms can last for 1,000 years — by the time the dust clears, allowing light from the accretion disk to penetrate, those important early stages are long over.

Something must have been strange about this particular stellar explosion.

“We think we must be dealing with a very little amount of ejecta,” Margutti said. “That’s why we can actually see the interior right away.”

This explanation would account for another strange feature of the Cow: It doesn’t carry the light signature of large amounts of nickel, the element that usually lends brightness to a supernova.

In this case, Margutti said, the Cow must be continuously powered by the object at its center — the black hole or neutron star shining through.

An alternative explanation suggests that the Cow came from a tidal disruption event — the dazzling destruction of a white dwarf star that passed too close to an incredibly massive black hole. The black hole’s gravity would break the star apart into a swirling stream of gas, generating an initial burst of light followed by months of glow in various wavelengths.

But the Cow does not come from a spot in its galaxy where such large black holes are usually found. So the mystery persists.

More clues may be out there, waiting to be found. Previous studies have pinpointed similar objects known as “fast luminous blue transients.” But scientists assumed that they were just a variation on the standard supernova, so none of those events were investigated as thoroughly as the Cow.

Astronomers will not be making that mistake again, Perley said.

“We should be able to find many more of these in the future, if we know what to look for,” he said. “And we think that we do now.”