You never know exactly when a central supermassive black hole is going to power up and start gobbling matter. Contrary to the popular view that these monsters are constantly devouring nearby stars and gas clouds, it turns out they spend part of their existence dormant and inactive. New observations from the European Space Agency’s XMM-Newton spacecraft opened a window on the “turn on event” for one of these monsters in a distant galaxy.
The supermassive black hole at the heart of the galaxy SDSS1335+0728 has been sleeping quietly for decades. Then, it suddenly lit up in 2019, and astronomers caught the outbursts in both optical and x-ray light. The galaxy, which lies some 300 million light-years away, was experiencing a wake-up call from its central black hole. Astronomers named this newly active galactic nucleus “Ansky”.
“When we first saw Ansky light up in optical images, we triggered follow-up observations using NASA’s Swift X-ray space telescope, and we checked archived data from the eROSITA X-ray telescope, but at the time we didn’t see any evidence of X-ray emissions,” said European Southern Observatory researcher Paula Sánchez Sáez. She and her team were the first to explore the activity at the black hole. Another team led by Lorena Hernández-García, a researcher at Valparaiso University, Chile, followed up on their work in 2024 by detecting X-ray bursts from the same region. “This rare event provides an opportunity for astronomers to observe a black hole’s behavior in real time, using X-ray space telescopes XMM-Newton and NASA’s NICER, Chandra, and Swift. This phenomenon is known as a quasiperiodic eruption, or QPE. QPEs are short-lived flaring events. And this is the first time we have observed such an event in a black hole that seems to be waking up,” explained Lorena.
ESA’s XMM-Newton satellite detected light from Ansky’s “wake-up” routine.
The Possible Sequence of Events at Ansky
What happened to light up Ansky in a QPE? Think about what happens as a black hole feeds. We know that the extreme gravitational pull of the black hole draws matter in, usually via a spinning accretion disk. It could be a star that wanders too close, for example. The strong gravitational influence of the black hole shreds the star, scattering its material throughout the disk. These are called “tidal disruption events” (TDEs).
That sounds great until you realize Ansky doesn’t seem to have destroyed a star. There’s no observational evidence. However, it has been experiencing recurring bursts of activity, which implies more than one event is occurring. It’s possible that a QPE could be caused by a star or some other object interacting with the accretion disk but not getting sucked in and creating a different kind of TDE.
Researchers went back to the drawing board to consider other possibilities. It could be that the accretion disk isn’t formed by a star, but is a result of gas being drawn into the black hole’s gravity well. The x-ray flares that the astronomers saw at Ansky suggest the disk could be experiencing energetic shocks created by the presence of something traveling through the it but not getting disrupted. Something on a headlong rush through an accretion disk would produce multiple events — sort of like a boat cutting through water. Those events would create the x-ray flares. “The bursts of x-rays from Ansky are ten times longer and ten times more luminous than what we see from a typical QPE,” says Joheen Chakraborty, a team member and PhD student at the Massachusetts Institute of Technology.
To get the x-rays, you need highly energetic events in the disk. “Each of these eruptions is releasing a hundred times more energy than we have seen elsewhere. Ansky’s eruptions also show the longest cadence ever observed, of about 4.5 days. This pushes our models to their limits and challenges our existing ideas about how these X-ray flashes are being generated.”
Some Caveats
In a paper released in Nature (see below), the team suggests continued follow-up observations of this and other similar events at supermassive black holes. That’s because the recurring x-ray flares at Ansky, which occur about every 25 days, aren’t well understood. They might be happening because the accretion disk has an eccentric inner section, and the whole thing is precessing. If that’s the case, an intruding object is passing through two different surfaces in the disk.
The best way to tell is to monitor future QPEs. That makes Ansky a perfect “real-time” test subject for various models of QPEs and helps astronomers create better models of these types of events. In addition, they may provide fodder for future gravitational wave studies.
“For QPEs, we’re still at the point where we have more models than data, and we need more observations to understand what’s happening,” says ESA Research Fellow and X-ray astronomer, Erwan Quintin.
“We thought that QPEs were the result of small celestial objects being captured by much larger ones and spiralling down towards them. Ansky’s eruptions seem to be telling us a different story. These repetitive bursts are also likely associated with gravitational waves that ESA’s future mission LISA [gravitational wave detector] might be able to catch. It’s crucial to have these x-ray observations that will complement the gravitational wave data and help us solve the puzzling behaviour of massive black holes.”
ESA’s LISA space-based interferometer array will be the first space-based laboratory to detect gravitational waves, particularly from AGNs such as Ansky. Courtesy ESA.
For More Information
From Boring to Bursting: a Giant Black Hole Awakens
Discovery of Extreme Quasi-period Eruptions in a Newly Accreting Massive Black Hole
ArXiv Version of Nature Paper