Swift confirms rare off-nuclear tidal disruption event, evidence for a wandering massive black hole

NASA’s Neil Gehrels Swift Observatory has helped astronomers confirm a rare tidal disruption event far outside the center of its apparent host galaxy, making it one of the clearest cases yet for a likely wandering massive black hole.

NASA said Monday that Swift observed the event, designated TDE 2025abcr, as it flared in the outskirts of galaxy WISEA J014656.04-152214.7, about 750 million light-years away. The event was first flagged by the Zwicky Transient Facility, or ZTF, in November 2025 as an unusual brightening, with the first spectrum supporting that classification taken on Nov. 5, 2025. Researchers found the flare was offset by 9.5 arcseconds from the galaxy’s center, or about 9.3 kiloparsecs in projection.

That offset is the key point. Tidal disruption events, or TDEs — eruptions produced when a black hole’s gravity tears apart a star — are usually found in or near galactic centers, where supermassive black holes are typically located and where most searches have concentrated. Finding one this far from a galaxy’s nucleus gives astronomers a way to spot otherwise hidden massive black holes outside galactic cores.

“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.

Stein is lead author of a paper published Monday in The Astrophysical Journal Letters. A preprint is also available on arXiv.

The researchers classified TDE 2025abcr as a TDE-H+He event, based on optical spectra showing broad hydrogen and helium lines. The paper adopts a peak date of Nov. 14, 2025, after the source brightened for about four weeks and then faded. At peak, its bolometric luminosity was estimated at 4.71 × 10^43 erg per second, which NASA said is roughly equal to the output of 10 billion suns. Its best-fit blackbody temperature was about 30,220 Kelvin, or roughly 30,000 degrees Celsius.

Swift supplied key follow-up observations beginning Nov. 9, 2025. Its X-ray Telescope detected soft X-rays from the source, with an inferred luminosity of about 2.75 × 10^42 erg per second in the 0.3-10 keV band. “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.

The black hole thought to have disrupted the star is estimated at about 10^6.09 times the mass of the sun, or roughly 1 million to 2 million solar masses. By contrast, the host galaxy’s central black hole is inferred to be far larger, on the order of 10^8.8 solar masses or more. That mismatch is one reason the authors argue the disruption likely did not come from the main galactic nucleus.

The leading explanation is that the flare came from a wandering black hole left in the galaxy’s outskirts after past mergers, though the authors also consider a black hole that was dynamically ejected from a nucleus. They caution that the case is not closed: deeper late-time observations could still reveal a very faint stripped dwarf galaxy or compact stellar system at the flare site.

The event was found using an off-nuclear version of the “tdescore” machine-learning classifier applied to ZTF alerts. “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.

Researchers say future surveys, including the Vera C. Rubin Observatory and NASA’s Nancy Grace Roman Space Telescope, should uncover more off-nuclear TDEs, opening a new way to study how galaxy mergers can leave black holes roaming far from their expected homes.