Title: Discovery of extreme quasi-periodic eruptions in a newly accreting massive black hole
Authors: Lorena Hernández-García, Joheen Chakraborty, Paula Sánchez-Sáez, Claudio Ricci, Jorge Cuadra, Barry McKernan, K. E. Saavik Ford, Patricia Arévalo, Arne Rau, Riccardo Arcodia, Erin Kara, Zhu Liu, Andrea Merloni, Gabriele Bruni, Adelle Goodwin, Zaven Arzoumanian, Roberto J. Assef, Pietro Baldini, Amelia Bayo, Franz E. Bauer, Santiago Bernal, Murray Brightman, Gabriela Calistro Rivera, Keith Gendreau, David Homan, Mirko Krumpe, Paulina Lira, Mary Loli Martínez-Aldama, Mara Salvato & Belén Sotomayor
First Author’s Institution: Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes (TITANS), Valparaíso, Chile; Millennium Institute of Astrophysics (MAS), Santiago, Chile; Instituto de Física y Astronomía, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile
Status: Published in Nature [closed access]
Picture this: a galaxy suddenly brightens in December 2019. Its light curve starts varying like an AGN, but it doesn’t emit any x-rays until February 2024, when it starts letting out brief (1.5 days), regular (about every 4.5 days) bright X-ray pulses. These pulses are called “quasi-periodic eruptions,” and the authors of today’s paper think they might be related to the new accretion disk around this (super)massive black hole.
What are QPEs?
Quasi-periodic eruptions, or QPEs, are regularly-timed (hence “periodic”) X-ray flares from an accretion disk around a supermassive black hole. They can repeat within as little as 2-3 hours or as long as a day. Each X-ray flare can last between ~15 minutes and a few hours. (Today’s paper found an object that lasted longer under both these metrics, which we’ll explore later). These properties are correlated so that longer-duration QPEs also have a longer period of “quiescence,” or low X-ray activity, between eruptions. Before this paper, there had only been eight QPEs ever observed, with the first one, GSN 069, observed in 2019. That means QPEs are brand new, and we’re only just starting to understand what they are.
The leading theory for QPEs is that they’re caused by something called an “extreme mass ratio inspiral.” The system starts with a supermassive black hole with an accretion disk. A stellar-mass object – typically a white dwarf, stellar-mass black hole, or some other stripped star – is orbiting the supermassive black hole. Importantly, the smaller object is not in the same plane as the accretion disk. Instead, it punches through the accretion disk, shocking the gas in the disk and superheating it, which emits X-rays. See Figure 1 for an illustration of this model.
This theory is supported by the recent discovery of an association between QPEs and tidal disruption events, or TDEs. TDEs are the disruption of stars that wander too close to a supermassive black hole. TDEs mostly occur in galaxies which recently shut off their star formation after a dramatic burst of star formation in the past billion years. Scientists think that starbursts are often caused by galaxy mergers, which may explain the increased rate of TDEs – as the merger occurs, it messes up the orbits of stars in each galaxy, which may send some unlucky stars careening towards the central supermassive black hole. Some of those stars would be disrupted as TDEs, leaving behind an accretion disk. Afterwards, the same messed-up dynamics might send a stellar-mass object into orbit through the disk, causing the QPE.
Why this one is special
The authors of today’s paper named their QPE “Ansky.” Ansky is special because it has a longer duration and recurrence time than any QPE observed before. It’s also the first time a QPE has been observed in a supermassive black hole just after it started accreting.
Researchers first started monitoring Ansky’s host galaxy back in 2019, when it suddenly got brighter in the Zwicky Transient Facility (which monitors the night sky for sudden brightness changes associated with astronomical transients). Soon after, its brightness started randomly varying, a telltale sign of an actively accreting supermassive black hole (active galactic nucleus, or AGN). AGN typically give off X-ray emission due to both their high temperatures and the many relativistic electrons in their atmospheres. However, this object wasn’t detected in X-ray surveys until 2024, nearly five years after the onset of optical emission. When it was, researchers noticed its regular eruptions and began monitoring for flares (see Figure 2).
Ansky’s duration and recurrence time are longer than have ever been observed in a QPE (Figure 3). Ansky’s flares last ~1.5 days, with about 4.5 days between the peak of each flare. Interestingly, it has a slightly longer (~6.5 day) gap every 25 days, which may tell us something about the system causing the QPE.
Model for the system
To understand why Ansky is different from other QPEs, the authors set up a model. Recall that the leading theory for a QPE is a stellar-mass object punching through the accretion disk of a supermassive black hole. In this case, the smaller object should be in an orbit around the larger one, so a longer time between peaks means it is further from the black hole. The duration of each eruption tells us something about the amount of mass ejected when the orbiter passes through the disk. Assuming the orbiter punches out a roughly steady volume of gas each time, more ejected mass (as we expect for a longer-duration flare) means the disk is denser. Most accretion disk models for supermassive black holes say that the disk gets denser with radius, so it makes sense that QPEs have longer duration times as their recurrence times get longer (see Figure 3).
Using the recurrence time of 4.5 days, the authors set up two models. In the first, the orbiting object is in a basically circular orbit, meaning it punches through the accretion disk twice per orbit. That means the period of the entire orbit is about nine days. Using observations of the accretion disk between flares, the authors determine that the central supermassive black hole should have a mass of around 10^6 solar masses. That gives an orbital semimajor axis of about 8.5 astronomical units (AU; one AU is the distance between the Earth and the Sun) . If this is the correct distance, the radius of the accretion disk (which must be at least 8.5 AU for the orbiter to punch through it) is much larger than would be expected for a disk from a previous TDE. Instead, this model would be more consistent with the disruption of a massive gas cloud.
In the second model, the authors assume that the orbiter only punches through the disk once per orbit, which could happen if the orbit is very eccentric. That would make the orbital period 4.5 days, leading to a semimajor axis of 5.3 AU. That would still be much farther than expected for a TDE disk.
To explain the extra delay between eruptions every 25 days, the authors propose that the accretion disk has a lower-density cavity that is precessing around the disk. That would require the disk to be about 16 AU. Every five or so orbits, the stellar-mass object would pass through the cavity instead of the disk itself, causing it to “miss” an eruption. The authors give the caveat that the disk would need to be warped or eccentric for this model to work, but without a more detailed simulation, this is the best model they have.
What now?
QPEs are weird objects that we still don’t fully understand. Why do they seem to happen more often in new accretion disks? Can they have longer or shorter timescales than the QPEs we’ve already found? As we find more, our models for accretion disks, the dynamics of galactic nuclei, and high-energy astrophysics will have to keep improving to keep up.
Astrobite edited by Amaya Sinha
Featured image credit: Hernández-García et al. (2025)
