Astronomers have discovered a star being torn apart by a massive black hole more than 1,000 parsecs from the center of its host galaxy. The tidal disruption event, TDE 2023mfm, was first discovered by ZTF and then studied with a wide range of telescopes, including Keck, Lick, Hubble, Swift, Chandra, XMM-Newton and the VLA. The discovery paper was posted to the arXiv preprint server on Sept. 28.
A midway snack
Massive black holes typically lurk at the center of massive galaxies. Recently, a population of “wandering” black holes, existing far from galactic centers, has been uncovered. They may end up in off-center regions because of gravitational-wave recoil, slingshot ejection, stalled orbital decay during galaxy mergers and long-lived massive black hole binaries or triples.
Finding massive black holes located away from the centers of galaxies can help astronomers understand the population and evolution of lesser-explored intermediate-mass black holes and massive black holes. But they are hard to find because they are not actively feeding, so they produce no steady, detectable signal.
But sometimes, stars that get too close to black holes are ripped apart in a tidal disruption event (TDE), which can produce a debris stream. This results in a bright flash across different wavelengths and reveals the location of the wandering black hole.
In this study led by Itai Sfaradi of the University of California, researchers present one such tidal disruption event, TDE 2023mfm, analyzed using optical and UV photometry with the Zwicky Transient Facility (ZTF), Hubble Space Telescope (HST) and Swift UVOT. In addition, they used optical spectroscopy with Keck and Lick observatories, X-ray observations with Chandra and XMM-Newton, and multifrequency radio observations with the Very Large Array.
The optical light curve reached a peak in 30 days and then slowly declined over 100 days. They fit the spectral energy distribution (SED), which shows how the source’s light is spread across wavelengths.
Using this distribution in the optical and ultraviolet bands, they inferred a temperature of around 20,000 Kelvin, and it remained this hot for more than a month after the peak. The event took place 1.08 kiloparsecs (roughly 3,500 light-years) from the galaxy’s center.
Multiwavelength check
The team did not find any X-ray emission from the TDE itself. A faint X-ray source was detected, but it most likely came from the galaxy’s nucleus rather than the TDE.
The lack of X-ray emission at the beginning is common among tidal disruption events, but a nondetection at later times is not as common. About 40% of TDEs found with optical telescopes become X-ray bright later. However, they say that this is “not unprecedented,” as some optically selected TDEs can remain very faint for years after the disruption event.
Its brightness and timing resemble common TDEs, and it closely matches another off-center event, TDE 2024tvd, in peak brightness, duration and host galaxy size. They detected a clear broad hydrogen line, classifying it as a TDE-H, a category whose spectra show only hydrogen emission lines. Together with its blue colors, its sustained high temperature and its off-center position, this supports its identification as a genuine TDE.
“… the clear broad hydrogen line in the host-subtracted optical spectrum, and the off-nuclear position observed both in the ZTF alerts and in multiple high-resolution optical, UV, and radio images, we conclude that TDE 2023mfm is an off-nuclear TDE-H,” the team says.
They used three methods to estimate the black hole’s mass at roughly 1 million to 10 million times the sun’s mass. “This is similar to many of the known TDEs,” they write.
Radio silence
Researchers explored scenarios for how this black hole could have ended up in that location.
It could be a remnant of a small galaxy that may have collided with the host galaxy. The galaxy shows no sign of a recent major merger, but a minor one can’t be ruled out, researchers explain. A small clump of stars—around 10 million to 100 million solar masses—sits at the black hole’s location, but it is too light for the black hole’s mass, so the small galaxy must have been heavily stripped of its stars.
“A different possible origin for this system is that the offset MBH was ejected from a triple MBH system due to gravitational wave recoil,” researchers explain the second scenario.
When three black holes interact, one gets flung out of the system by gravitational wave recoil. The researchers estimate this could account for about 1% of TDEs, so with around 250 known optical TDEs, one such case is plausible by the numbers.
Another possibility is that two black holes merged and the gravitational waves they emitted kicked the merged black hole out of the galactic center, leaving a single black hole outside the nucleus.
Both ideas are ruled out for the same reason: The galaxy’s center clearly hosts its own black hole, shown by a faint active galactic nucleus (AGN). The galaxy therefore has two black holes, which makes the minor-merger scenario plausible.
“The radio emission, on the other hand, tells a different story,” the researchers write. Radio telescopes saw nothing from TDE 2023mfm until a faint signal finally showed up at about 991 days (roughly 2.7 years), and it changed only slowly after that.
TDE 2023mfm looks much like its off-center cousin TDE 2024tvd in visible and ultraviolet light, but its radio behavior is very different: faint, late and slowly evolving. Whether that comes from a missing early outflow, a different surrounding gas environment or the viewing angle is still an open question. Further radio observations may help settle it.
“If radio emission is also detected from the off-nuclear TDE, this can provide a confirmation for the off-nuclear MBH and a precise measurement between the two MBHs,” they note.
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Publication details
Itai Sfaradi et al., An Off-Nuclear Tidal Disruption Event Discovered At Late Times: The Case Of TDE 2023mfm, arXiv (2026). DOI: 10.48550/arxiv.2609.35998
Journal information:
arXiv
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Shredded star reveals a massive black hole wandering far from its galaxy’s center (2026, October 16)
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