Their orbit periodically takes them through a cloud of gas, triggering flares.
What happens when a gargantuan cloud of gas swallows a pair of monster black holes with their own appetites? Feasting on the gas can cause some weird (heavenly) bodily functions.
AT 2021hdr is a binary supermassive black hole (BSMBH) system in the center of a galaxy 1 billion light-years away, in the Cygnus constellation. In 2021, researchers observing it using NASA’s Zwicky Transient Facility saw strange outbursts that were flagged by the ALerCE (Automatic Learning for the Rapid Classification of Events) team.
This active galactic nucleus (AGN) flared so brightly that AT 2021hdr was almost mistaken for a supernova. Repeating flares soon ruled that out. When the researchers questioned whether they might be looking at a tidal disruption event—a star being torn to shreds by the black holes—something was still not making sense. They then compared observations they made in 2022 using NASA’s Neil Gehrels Swift Observatory to simulations of something else they suspected: a tidal disruption of a gas cloud by binary supermassive black holes. It seemed they had found the most likely answer.
“The variations in AT 2021hdr cannot be easily explained by any of the mechanisms usually associated to SMBHs,” the team said in a study recently published in Astronomy and Astrophysics.“However, we find that the behavior of AT 2021hdr broadly fits with models of the disruption and accretion of a gas cloud by a BSMBH.”
Unusual symptoms
What made AT 2021hdr a bad match for some of the phenomena it was initially thought to be? Like other supermassive black holes in active galactic nuclei, both of the black holes in this system have accretion disks of material that spirals inward as it is pulled by their powerful gravity. The accretion disk glows as friction and collisions slows the material enough to be devoured by the black hole.
But AT 20221hdr doesn’t just glow. It flares every 60 to 90 days.
The first thing that the researchers needed to find out was whether these flares were just variations in AGN behavior. The light intensity of an AGN can fluctuate over time as a result of changes in processes occurring close to a supermassive black hole, or in this case, a supermassive black hole binary. The flares of AT 2021hdr were not intense enough to indicate the presence of jets, which eliminated one potential source of variability.
Because the team observed sudden increases in X-rays, they knew that the system was flaring. But in cases of random AGN flares known as quasi-periodic eruptions or QPEs, there are periods of quiescence, or inactivity. There was no quiescence observed in AT 2021hdr, and its flares were visible in UV and optical light in addition to the X-ray part of the spectrum.
Tidal disruption of a star was also suggested as a cause of the flaring behavior in AT 2021hdr. Tidal disruption events (TDEs) happen when an unfortunate star passes close enough to a black hole to be shredded by the black hole’s gravity. Most TDEs do not flare, but one was recently observed flaring regularly. Could that also apply here?
Has to be gas
The problem with the star hypothesis is that stars are not nearly as large as whatever was being consumed by this system. The strength of the flares seemed to imply that whatever the black holes were eating was much larger. After ruling out several phenomena as the cause behind AT 2021hdr’s flares, the researchers landed on the possibility that fit their observations.
“In addition to stars, gas clouds can also be disrupted by SMBHs and their binaries,” they said in the same study. “The key difference is that the clouds can be comparable to or even larger than the binary separation, unlike stars, which are always much smaller. “
Looking at the results of a previous study that numerically modeled this type of situation also suggested a gas cloud. Just like the hypothetical supermassive black hole binary in the model, AT 2021hdr would accrete large amounts of material every time the black holes were halfway through orbiting each other and had to cross the cloud to complete the orbit—their gravity tears away some of the cloud, which ends up in their accretion disks, every time they cross it. They are now thought to take in anywhere between three and 30 percent of the cloud every few cycles. From a cloud so huge, that’s a lot of gas.
The supermassive black holes in AT 2021hdr are predicted to crash into each other and merge in another 70,000 years. They are also part of another merger, in which their host galaxy is gradually merging with a nearby galaxy, which was first discovered by the same team (this has no effect on the BSMBH tidal disruption of the gas cloud).
How the behavior of AT 2021hdr develops could tell us more about its nature and uphold or disprove the idea that it is eating away at a gaseous cloud instead of a star or something else. For now, it seems these black holes don’t just get gas from what they eat—they eat the gas itself.
Astronomy & Astrophysics, 2024. DOI: 10.1051/0004-6361/202451305
Elizabeth Rayne is a creature who writes. Her work has appeared on SYFY WIRE, Space.com, Live Science, Grunge, Den of Geek, and Forbidden Futures. She lurks right outside New York City with her parrot, Lestat. When not writing, she is either shapeshifting, drawing, or cosplaying as a character nobody has ever heard of. Follow her on Threads and Instagram @quothravenrayne.
