Title: The Wandering Supermassive Black Hole Powering the off-nuclear TDE AT2024tvd

Authors: M. Guolo, A. Mummery, S. van Velzen, M. Nicholl, S. Gezari, Y. Yao, K. C. Chambers, T. de Boer, M. E. Huber, C.-C. Lin, T. B. Lowe, E. A. Magnier, G. Paek, and R. Wainscoat

First Author’s Institution: Bloomberg Center for Physics and Astronomy, Johns Hopkins University, USA

Status: Submitted to the Astrophysical Journal Letters, open access on arXiv

A Star Gets Eaten in the Wrong Neighborhood

We find supermassive black holes at the centers of most large galaxies. They end up there because of how galaxies form: as matter collapses and merges over time, material sinks to the gravitational center, and the black hole settles in.

So, what happens when we catch a star being ripped apart by a supermassive black hole that is not at the center of its galaxy?

That is exactly the puzzle posed by AT2024tvd, a tidal disruption event, or TDE, spotted about 0.8 kiloparsecs (roughly 2,600 light-years) from the center of a massive galaxy located 600 million light years away from us (see Figure 1).  

We see TDEs when a black hole tears apart a star that gets too close to it. It happens because the black hole’s gravity pulls harder on the near side of the star than the far side, stretching it until it comes apart. The stellar debris then forms a hot accretion disk, a ring of material swirling around the black hole that spirals inward and releases a burst of electromagnetic energy across many wavelengths – from visible light to X-rays. These events are valuable to astronomers because they briefly light up black holes that would otherwise be invisible, giving us a rare window to measure their properties. Since supermassive black holes live at galactic centers, that is also where we expect TDEs to happen, which makes a TDE found away from a galactic center a rare and puzzling find. The galactic center is often called the nucleus of a galaxy, so astronomers refer to these displaced events as “off-nuclear” TDEs. AT2024tvd is only the third known off-nuclear TDE, and this paper makes a strong case that it is the most remarkable one yet.

Figure 1: A color image of AT2024tvd from HST and JWST. The yellow dot inside the square marks the galaxy’s center, while the TDE is the white dot, which is visibly offset to the upper left, about 2,600 light-years away.

Measuring Mass using Light

To figure out the mass of the black hole responsible for AT2024tvd, the authors needed to get creative. You cannot weigh a black hole directly, so astronomers have to work backward from what they can see. The key idea is that a black hole’s mass controls how its accretion disk behaves. A more massive black hole produces a larger, cooler disk, while a less massive one produces a smaller, hotter disk. By measuring how bright the disk is at different wavelengths and how hot it gets, you can figure out how massive the black hole must be.

They did this by modeling the light the event produced across many wavelengths. They combined data from several telescopes: the Zwicky Transient Facility (ZTF), the Swift observatory, Pan-STARRS, and two rounds of high-quality X-ray data from XMM-Newton. TDE light curves go through different phases. The early flare in visible and ultraviolet (UV) light is bright but complicated, and the physical processes behind it are not fully understood. But after a few hundred days, TDEs settle into a quieter “plateau phase,” where the UV and visible light come directly from the accretion disk. At this stage, the emission follows well-understood physics, and astronomers can model it reliably.

The authors used a model called kerrSED, which describes spectral energy distribution (SED) of a spinning black hole’s accretion disk (kerr). It accounts for the disk’s temperature, its physical size, the black hole’s spin (how fast it rotates), and the angle at which we are viewing the system. It also accounts for a process called Comptonization, where hot electrons near the black hole boost lower-energy photons (particles of light) up to X-ray energies. By fitting this model to the observed light at multiple wavelengths simultaneously, the authors could pin down the disk properties and extract the black hole mass. The result was a clean fit: the disk model alone could explain all the observed light during the plateau phase, with nothing significant left over (see Figure 2).

Figure 2: The brightness of AT2024tvd measured across a wide range of wavelengths, after correcting for absorption and the galaxy’s motion. The colored points show individual measurements from different telescopes, while the shaded contours represent the best-fit disk model and its uncertainty. Left: the early X-ray data, which is well explained by emission from the hot inner accretion disk. Right: the later data covering both visible/UV light and X-rays, all consistently explained by the same disk model.

Not an Intermediate, but a Supermassive Black Hole

From their fit, the authors found a black hole mass of about one million solar masses. Black holes above about 100,000 solar masses are considered supermassive, while those below that threshold are called intermediate-mass black holes. So, this puts AT2024tvd in the supermassive category. This matters because the two previously known off-nuclear TDEs, called 3XMM J2150-05 and EP240222a, were both powered by intermediate-mass black holes. Those black holes were found inside small, dense collections of stars called ultra-compact dwarf galaxies that were orbiting larger host galaxies.

AT2024tvd is different. When the authors looked at deep images of the location where the TDE happened, they found no star cluster or small galaxy there. Whatever group of stars once surrounded this black hole has been almost entirely pulled apart by the gravity of the much larger parent galaxy. The ratio of the black hole’s mass to the mass of any remaining stars around it is extreme: greater than 3%, which is far above what we normally see. This is the signature of a “wandering” supermassive black hole, one that was brought in during a past galaxy merger and has been slowly sinking toward the center of its new host ever since, losing its surrounding stars along the way.

The authors compared AT2024tvd to other TDEs using established relationships between accretion disk properties and black hole mass (Figure 3). In terms of its disk temperature, luminosity, and inferred mass, AT2024tvd behaves like a typical nuclear TDE powered by a supermassive black hole. Although, when placed on the black hole mass versus host stellar mass relation, it stands out as a strong outlier. The black hole mass is far too large for the small amount of surrounding stellar mass detected at its location.

Figure 3: AT2024tvd (yellow star) plotted on the relationship between black hole mass and host galaxy stellar mass. Red squares show nearby galaxies with dynamically measured black hole masses plotted against galaxy bulge stellar mass, while blue squares show the relation using total galaxy stellar mass. Purple points and green diamonds represent nuclear TDE host galaxies with black hole masses inferred from different TDE modeling techniques. Yellow diamonds mark the two previously known off-nuclear TDEs. AT2024tvd stands out as a clear outlier, with a very high black hole mass compared to the upper limit on any surrounding stellar mass.

The Big Picture

This discovery matters for several reasons. It shows that off-nuclear TDEs are not limited to intermediate-mass black holes sitting in small satellite galaxies. Some are powered by fully supermassive black holes that have been displaced from their original galactic centers. It also shows that TDE modeling, when done carefully during the plateau phase of the light curve (when the emission is dominated by well-understood accretion disk physics), can provide reliable black hole masses on its own, without assuming any relationship between the black hole and its host galaxy. This is particularly important for wandering black holes, where those relationships do not apply.

Looking ahead, upcoming surveys like Vera C. Rubin Observatory’s Legacy Survey of Space and Time (LSST) are expected to find many more off-nuclear TDEs. Combined with X-ray follow-up from telescopes like XMM-Newton, these events could become our main tool for mapping out the population of displaced black holes in the nearby universe. One disrupted star at a time, we are starting to find black holes that theory told us should exist but that had, until now, stayed hidden.

Astrobite edited by Kelsie Taylor, Veronika Dornan

Featured image credit: Serat Saad