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."

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.