Then again, the strange body could be a rare type of supernova or an oddball "midsize" black hole—more massive than black holes born when single stars explode but "lighter" than the supermassive ones at the centers of galaxies.
"All three of those [options] are exotic and have something peculiar to them," said study co-author Peter Jonker, an astronomer with the Netherlands Institute for Space Research in Utrecht.
Off-center Black Holes Wanted
Jonker and his colleagues found the mystery object while on the hunt for off-center supermassive black holes that are thought to form when two galaxies merge. (Related: "Colossal Four-Galaxy Collision Discovered.")
Most, if not all, galaxies are thought to have supermassive black holes at their cores. Recent computer simulations suggest that when two galaxies merge, so do their central black holes.
But the newly formed black hole combo "actually receives a kick" from gravitational forces generated by the galactic merger, Jonker said. The kick, according to the models, "launches this newly formed black hole out of the center of the galaxy." (See "Hundreds of 'Rogue' Black Holes May Roam Milky Way.")
Sorting through archived data from the Chandra X-ray Observatory, the team found an interesting candidate in a galaxy half a billion light years away from Earth. The extremely bright x-ray object is about ten thousand light-years from its galactic center.
Based on the Chandra data, however, the astronomers couldn't rule out the possibility that the newfound object actually lies behind the galaxy in question.
So the team compared their x-ray information with archived optical data from the Hubble Space Telescope. They found that the mystery object emits a bright blue light in visible wavelengths. (See NASA astronomer's picks for the top Hubble pictures of the past 20 years.)
"If you look through [a] galaxy toward something in the background, you go through a layer of material"—interstellar dust—"that preferentially takes out the blue light," so you wouldn't see it, Jonker explained.
The object's blue hue helps confirm that it belongs to the galaxy in question, the team reports in a paper accepted for publication in the Monthly Notices of the Royal Astronomical Society.
"Having ruled out the background source, then the question is, OK, what then can it be?" Jonker said.
Rare Supernova, or Even Rarer Black Hole?
One possibility is that the object is "a very strange supernova that we don't know very much about" called a Type IIn supernova, Jonker said.
In general, a Type II supernova happens when a massive star runs out of fuel and its core collapses. Such supernovae get different letter designations based on the characteristics of hydrogen in their light signatures, or spectra. Type IIn supernovae have narrow spectral lines of hydrogen and glow the brightest in x-rays.
A popular explanation is that the light we see isn't coming from the supernova itself, but from interactions between material ejected by the explosion and a cloud of charged gas that surrounded the dying star. But these supernovae are rare and are still poorly understood.
If the newfound object is a Type IIn supernova, that would mean Hubble saw the star before it exploded, while Chandra's x-ray eyes saw the aftermath of the blast, Jonker said, because the data come from two different time periods. (Video: Watch the first 3-D simulation of the start of a Type II supernova.)
What's odd, Jonker added, is that the supernova would have been bright enough for astronomers on Earth to detect—but no one did.
Another possibility is that the object is an intermediate-mass black hole, a type of black hole that astronomers think should exist but that no one has officially confirmed. If that's the case, the object is a black hole with a mass about ten thousand times that of the sun.
Compared with the other x-ray sources that might be black hole "middleweights," however, the newfound object is on the brighter end of the spectrum, Jonker said.
An ejected supermassive black hole, meanwhile, would drag with it everything in its gravitational sphere of influence, he said. This "baggage" would be sufficient to power a black hole for tens of millions of years, allowing it to shine brightly in x-rays.
"So that is a very intriguing possibility," Jonker said.
Supernova Is a Best Bet
Astronomer Avi Loeb, who was not involved in the study, thinks the best explanation of the three is that the object is some variety of supernova.
Additional x-ray observations could confirm this by showing if the object's x-ray emission strength has declined, as would be expected from the cooling debris of an exploded star, said Loeb, who directs the Institute for Theory and Computation at Harvard University.
While the object might be too bright as intermediate-mass black holes go, it's too dim to be an ejected supermassive black hole, Loeb thinks. If it is a black hole at all, "a medium-mass black hole would therefore be more natural," he said.
"At this time, it would be prudent to get more data rather than speculate further about the nature of this mystery source," he added.
Study co-author Jonker said his team plans to do just that.
"We've asked for more observations," he said, "since all this is dependent on an object we found in an archival study."