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VLA Observations Confirm AT 2023mfm as an Off-nuclear Tidal Disruption Event

High-resolution VLA radio observations confirm that the transient AT 2023mfm is an off-nuclear tidal disruption event located 1.06 kpc from its host galaxy's nucleus, validating the efficacy of radio imaging in identifying such events and revealing hidden off-nuclear massive black holes.

Original authors: Wenkai Li, Collin T. Christy, Kate D. Alexander, Itai Sfaradi, Xinya Huang, Ning Jiang, Tanmoy Laskar, Andrew Mummery, Noah Franz, Adelle J. Goodwin, Walter W. Golay, Raffaella Margutti, Ryan Chornock

Published 2026-06-08

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Original authors: Wenkai Li, Collin T. Christy, Kate D. Alexander, Itai Sfaradi, Xinya Huang, Ning Jiang, Tanmoy Laskar, Andrew Mummery, Noah Franz, Adelle J. Goodwin, Walter W. Golay, Raffaella Margutti, Ryan Chornock, Jiazheng Zhu, Sjoert van Velzen, Yvette Cendes, Wenbin Lu, Jimmy Lynch

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a massive galaxy as a bustling city. Usually, the most powerful figure in this city—the supermassive black hole—sits right in the center, like a mayor in a grand city hall. But sometimes, due to cosmic "traffic accidents" (like galaxies crashing into each other), this black hole gets knocked out of the mayor's office and ends up living in a suburb, far away from the city center.

This paper is about a cosmic event called AT 2023mfm, which scientists suspected was a "star-eating" incident (a Tidal Disruption Event, or TDE) happening in one of these suburbs, not the city center.

Here is the story of how they proved it:

The Mystery: A Star in the Wrong Place

A few years ago, a telescope called the Zwicky Transient Facility (ZTF) spotted a bright flash of light. It looked like a star had been torn apart by a black hole. However, when astronomers looked at the map, the flash wasn't happening in the middle of the galaxy. It was offset, like a firework exploding in a backyard instead of the town square.

The team had a hunch: "Maybe there's a hidden black hole living in that backyard, and it just ate a star." But looking at the galaxy with optical telescopes (which see visible light) wasn't enough to be 100% sure. The backyard and the city center looked too close together to tell them apart clearly.

The Detective Work: Using Radio "X-Ray Vision"

To solve the mystery, the team used the Very Large Array (VLA), a giant radio telescope that acts like a high-resolution camera for invisible radio waves. Think of this as switching from a blurry security camera to a sharp, 4K lens that can see tiny details.

They pointed the VLA at the galaxy and found two distinct radio "beacons" (sources of radio energy):

The City Center Beacon: One signal came from the exact center of the galaxy. This is likely the galaxy's main black hole, which is still there, behaving normally.
The Backyard Beacon: A second signal came from a spot 0.65 arcseconds away (about 1,000 light-years in real distance). This spot matched perfectly with where the star explosion (AT 2023mfm) was seen in visible light.

The Verdict

The paper confirms that the "Backyard Beacon" is indeed the black hole that ate the star.

The Proof: The radio signal from the explosion is clearly separated from the galaxy's center. It's not a trick of the light; there are two distinct objects.
The Story: This suggests that a smaller galaxy merged with this big one. The smaller galaxy brought its own black hole, which got knocked off-center during the crash. That displaced black hole is now living in the "suburbs" of the big galaxy, and it recently had a snack (the star).

Why This Matters

This discovery is like finding a hidden house in a neighborhood you thought was empty. It proves that high-resolution radio imaging is a powerful tool for finding these "drifting" black holes that optical telescopes might miss. It confirms that AT 2023mfm is a rare, off-center star-eating event, adding to our understanding of how galaxies and their black holes move and interact after crashing into one another.

In short: The team used a super-sharp radio telescope to prove that a star was eaten by a black hole that was living in the "suburbs" of a galaxy, not in the "city center," confirming that black holes can get knocked out of their homes during cosmic collisions.

Technical Summary: VLA Observations Confirm AT 2023mfm as an Off-nuclear Tidal Disruption Event

Problem and Context
While most massive galaxies host central massive black holes (MBHs), theoretical models predict that MBHs can reside off-nucleus due to galaxy mergers, gravitational-wave recoil, or three-body interactions. These displaced MBHs can produce off-nuclear tidal disruption events (TDEs) when stars are scattered into their tidal radii. Although several off-nuclear TDE candidates have been identified in X-ray and optical surveys (e.g., 3XMM J215022.4-055108, AT 2024tvd, AT 2025abcr), confirming their off-nuclear nature and distinguishing them from line-of-sight coincidences or unrelated transients remains a challenge. High-resolution radio imaging is required to spatially resolve the transient from the host galaxy nucleus and validate these candidates.

Methodology
The authors conducted a systematic astrometric analysis of reported TDEs to identify high-confidence off-nuclear candidates. For AT 2023mfm, originally discovered by the Zwicky Transient Facility (ZTF) on 2023 June 28, the team compared transient positions against archival host-galaxy centroids from the Sloan Digital Sky Survey (SDSS), Pan-STARRS1 (PS1), and the DESI Legacy Imaging Surveys (LS). Sources separated from the host centroid by more than the instrument-specific empirical astrometric threshold were flagged as candidates.

To confirm the nature of AT 2023mfm, the authors utilized the NSF Karl G. Jansky Very Large Array (VLA) in A-configuration. Observations were taken on 2026 April 11 (1018 days post-discovery) in the C-band (6 GHz), with additional detections in X-band (10 GHz), Ku-band (15 GHz), L-band (1.5 GHz), and S-band (3 GHz). Data were reduced using the Common Astronomy Software Applications (CASA) package and imaged with tclean. Source positions and flux densities were measured using imfit, fitting elliptical Gaussians fixed to the synthesized-beam size. Absolute positional uncertainties included a conservative systematic term of 0.0515″ added in quadrature to the statistical errors.

Key Results

Resolution of Dual Sources: The high-resolution C-band (6 GHz) VLA image resolved two distinct radio point sources separated by $0.651 \pm 0.036''$ $0.651 \pm 0.03 6^{''}$ ( $1.06 \pm 0.06$ $1.06 \pm 0.06$ kpc at $z=0.087$ $z = 0.087$ ).
- Northwestern Source: Consistent with the host-galaxy nucleus (approximated by SDSS/PS1/LS centroids). It has a flux density of $60 \pm 6$ µJy and a luminosity of $\nu L_\nu = (6.8 \pm 0.7) \times 10^{37}$ erg s $^{-1}$ . It is detected across C, X, and Ku bands with a spectral index of $0.25 \pm 0.13$ , consistent with compact AGN-like emission.
- Southeastern Source: Consistent with the ZTF/PS1 optical TDE position. It has a flux density of $46 \pm 9$ µJy and a luminosity of $\nu L_\nu = (5.2 \pm 1.0) \times 10^{37}$ erg s $^{-1}$ . It is detected in C and X bands and tentatively detected in Ku-band ( $\gtrsim 3\sigma$ ).
Astrometric Confirmation: The southeastern radio source aligns with the optical TDE position within $1\sigma$ uncertainties, while the northwestern source aligns with the host nucleus. The separation is significantly larger than the $3\sigma$ transient-position ellipse derived from ZTF data.
Contamination Checks: The authors ruled out contamination by unrelated transients by:
- Comparing pre- and post-optical bump positions in the ZTF light curve, finding a negligible difference ( $0.057''$ ).
- Conducting a $5''$ search around the off-nuclear radio source, which revealed no other ZTF alerts or TNS-reported transients.
Host Properties: The host galaxy has a stellar mass of $\sim 10^{10.7} - 10^{11.0} M_\odot$ , consistent with massive, old hosts of other known off-nuclear TDEs. Archival imaging suggests a rich tidal/merger history with possible dwarf/satellite structures.

Significance and Claims
The paper claims that these VLA observations definitively confirm AT 2023mfm as an off-nuclear TDE. The spatial resolution of the radio sources demonstrates that the transient is physically displaced from the host nucleus, validating the candidate status derived from optical astrometry.

The authors argue that the data favor a minor-merger origin for the displaced MBH, rather than gravitational-wave recoil of the central black hole. This conclusion is based on:

The presence of a compact, AGN-like nuclear source, implying the central black hole remains in place.
The relatively low peak optical luminosity ( $\sim 2 \times 10^{43}$ erg s $^{-1}$ ), which suggests a lower MBH mass than expected for the host galaxy's total stellar mass, consistent with a satellite MBH that has not yet merged with the central nucleus.

The study highlights the power of high-resolution radio imaging to validate off-nuclear TDE candidates and reveal hidden off-nuclear MBHs, contributing to the growing sample of such events that link individual transients to galaxy-MBH assembly histories.

The Mystery: A Star in the Wrong Place

The Detective Work: Using Radio "X-Ray Vision"

The Verdict

Why This Matters

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