Catastrophic encounters between stars and massive black holes usually take place in the nuclei of galaxies, but not always. Researchers recently reported on the brightest-ever radio emission from an off-nuclear tidal disruption event caused by a wandering or recoiling black hole.
Signature of a Roaming Black Hole
Tidal disruption events occur when a star ventures too close to a massive black hole. The tidal forces of the black hole stretch the star until it’s partially or entirely disrupted, sometimes causing jets or outflows to spray from the shredded star. One thing that often distinguishes a tidal disruption event from the sea of other possible transients is the location, close to the nucleus of a galaxy.
But not all tidal disruption events happen in the center of a galaxy. In rare cases, a massive black hole roaming elsewhere in a galaxy may encounter a star, sending out a tell-tale signal in an unexpected location.
Radio observations of AT 2024tvd on two dates after its optical discovery. The tidal disruption event was not detected at 88 days post-discovery (left) and outshone the center of its host galaxy on 160 days post-discovery (right). Click to enlarge. [Sfaradi et al. 2025]
Radio Reconnaissance
Less than three months after AT 2024tvd was discovered, Itai Sfaradi (University of California, Berkeley) and collaborators launched a months-long radio-wavelength observing campaign using the Very Large Array, the Atacama Large Millimeter/submillimeter Array, the Arcminute Microkelvin Imager Large Array, the Allen Telescope Array, and the Submillimeter Array. Radio observations are critical for investigating jets and outflows from tidal disruption events.
The observations, which spanned centimeter and millimeter wavelengths, revealed two emission peaks from the tidal disruption event. The first peak occurred roughly 131 days after the event was discovered, and the second followed at day 194.
Demonstration of the fast evolution of AT 2024tvd’s radio emission (red and orange stars) compared to other radio-bright tidal disruption events (other symbols). [Sfaradi et al. 2025]
Prompt or Delayed, Outflow or Jet?
To understand the origin of the fast-evolving, extremely bright radio emission from AT 2024tvd, Sfaradi’s team modeled the emission that would arise from outflows and jets. For both wide-angle outflows and narrow jets, the team considered both prompt — arising simultaneously with the event’s optical detection — and delayed sources.
The team’s modeling highlighted several possible scenarios. In the first, both bright radio peaks arose from a single outflow that was launched about 84 days after the star met its doom. The double-peaked behavior is due to the outflow interacting with a complex distribution of material surrounding the black hole. It’s also possible for the two peaks to arise from separate outflows or jets, one launched around 84 days and the other around either 170 or 190 days, depending on whether the second source is a mildly relativistic outflow or a relativistic jet.
Sfaradi and collaborators posited that AT 2024tvd’s unusual radio behavior could be due to its off-nuclear location, but they acknowledged that this event might simply occupy a region of tidal disruption event parameter space that had yet to be explored. Sensitive interferometric or polarimetric observations may reveal more about how AT 2024tvd interacts with its environment, helping to illuminate the nature of this rare event.
Citation
“The First Radio-Bright Off-Nuclear TDE 2024tvd Reveals the Fastest-Evolving Double-Peaked Radio Emission,” Itai Sfaradi et al 2025 ApJL 992 L18. doi:10.3847/2041-8213/ae0a26