"We were looking for these star-shredding events as a way to find otherwise invisible supermassive black holes wandering away from the galactic cores where they usually reside," said Robert Stein, a research fellow at The University of Maryland, College Park and NASA's Goddard Space Flight Center in Greenbelt, Maryland. "With this discovery, which is one of just a couple that have been confirmed so far, we've validated a new technique and can use it to hunt for more."
A paper describing the results, led by Stein, was published on July 27 in The Astrophysical Journal Letters.
A Star Shredded by a Hidden Black Hole
The discovery began with an exceptionally bright flare produced when a star wandered too close to a massive black hole and was ripped apart by its intense gravity. Astronomers call this type of event a tidal disruption event.
The black hole responsible has a mass roughly 1 million times that of the Sun. Its presence was first suspected in November 2025, when ZTF (Zwicky Transient Facility), a sky survey operated by the Palomar Observatory in Southern California, detected an unusual burst of light in a galaxy about 750 million light-years from Earth.
"Out of the half million flashes ZTF detects each night, our new artificial intelligence algorithm automatically recognized a flare that looked a lot like a tidal disruption event, despite its unusual location in the outskirts of a galaxy," Stein said.
For several months, the flare became brighter in ultraviolet light than the entire galaxy surrounding it. At its peak, it temporarily radiated with the brilliance of about 10 billion suns.
Swift Confirms the Extreme Cosmic Flare
Additional observatories examined the source after the initial ZTF detection. The SOAR (Southern Astrophysical Research) telescope in Chile studied its spectrum and found features consistent with a tidal disruption event.
Astronomers then turned to NASA's Swift Observatory, which can examine wavelengths that cannot be detected by telescopes on the ground. Swift's UVOT (Ultraviolet/Optical Telescope), for example, measured the flare's temperature at about 54,000 degrees Fahrenheit (30,000 degrees Celsius).
"The combination of all this data helped us rule out other explanations and confidently say it's a tidal disruption event, despite its strange location," said Jonathan Carney, a doctoral student at the University of North Carolina at Chapel Hill, who took the first spectra that supported the flare's interpretation as a tidal disruption event.
Hidden Heavyweights at Galactic Centers
Nearly every galaxy in the universe appears to contain a supermassive black hole at its center. Roughly once every 100,000 years, a star in a given galaxy may stray close enough to one of these black holes to be torn apart in a tidal disruption event.
Although such events are extremely uncommon within an individual galaxy, astronomers monitor millions of galaxies in an effort to find them. Current surveys typically detect about 30 tidal disruption events across the universe each year.
Before 2024, every confirmed example had been found in a galactic core. That was partly because astronomers concentrated their searches there, since all known supermassive black holes had been located at galaxy centers. A tidal disruption event also requires a very massive black hole because the gravitational pull of lighter black holes isn't strong enough.
That assumption began to change when scientists detected signs of a star being destroyed 2,600 light-years from the center of its host galaxy. The discovery encouraged astronomers to expand their searches beyond galactic cores.
The newly identified event is even more extreme. It occurred more than 30,000 light-years from the center of its galaxy.
How Did the Black Hole Get There?
The black hole's unusual location raises a major question about its past.
"It must have originated in a galaxy's center, but not the one it's in the outskirts of now," Stein said. "We think the host galaxy's supermassive black hole is still at its core, but the one eating the star could have started off in a small galaxy that merged with the big one we see today."
Researchers have proposed two possible explanations.
In one scenario, three or more galaxies merged. Their central supermassive black holes may then have become locked in a gravitational struggle that launched the lightest black hole toward the edge of the combined galaxy.
Another possibility is that a dwarf galaxy is still in the process of merging with the larger system. As stars from the dwarf galaxy entered the larger galaxy, one of them may have passed too close to the dwarf galaxy's own supermassive black hole.
"Further discoveries could reveal the origin of this apparent 'orphan' black hole," Stein said. "The key science question we want to answer is: How common are wandering black holes?"
Swift Awaits an Orbit Boost
Astronomers may soon be able to answer that question by finding more displaced black holes.
"Pointed science observations with Swift's UVOT and XRT (X-Ray Telescope) instruments are temporarily suspended as the mission awaits an orbit boost, which is planned for this summer," said co-author S. Bradley Cenko, Swift's principal investigator at NASA Goddard.
Swift's primary mission lasted from 2004 to 2006, but the spacecraft has continued observing the changing universe for more than 20 years. Atmospheric drag is now gradually pulling it closer to Earth. Raising Swift into a higher orbit could keep it operating for even longer.
"Once it resumes normal operations, Swift could continue searching for more examples of out-of-place black holes."
A New Search for Wandering Black Holes
In the years ahead, astronomers plan to apply the same detection method to observations from the newly operational Vera C. Rubin Observatory, jointly funded by the U.S. Department of Energy and National Science Foundation, in Chile and NASA's upcoming Nancy Grace Roman Space Telescope.
"Rubin's wide, deep surveys will reveal a much larger sample of tidal disruption events than current observatories are capable of collecting, including ones that are off-center," Carney said. "And Roman's space-based surveys will extend the current search zone by seeing ones that are farther away, looking back through 9 billion years of cosmic history."
Combining data from Rubin, Roman, Swift, and ground-based observatories could allow scientists to identify many more wandering black holes. Together, these discoveries may eventually help astronomers build the most complete census yet of the universe's enormous black holes. %!s()
