What happens when an enormous cloud of gas encounters a pair of ravenous supermassive black holes?

The result is a cosmic spectacle that has left astronomers bewildered.

Discovery and Observation

Located 1 billion light-years away in the Cygnus constellation, AT 2021hdr is a binary supermassive black hole (BSMBH) system that has recently attracted intense scientific scrutiny. Discovered in 2021 by researchers using NASA’s Zwicky Transient Facility, the system exhibited strange outbursts that caught the attention of the ALerCE (Automatic Learning for the Rapid Classification of Events) team, leading to groundbreaking findings.

The Nature of the Flares

The flares emitted by AT 2021hdr were so intense that they nearly resembled a supernova explosion. However, repeating flares quickly eliminated that possibility. Researchers initially speculated about a tidal disruption event—a catastrophic scenario in which a star is torn apart by the gravitational forces of the black holes—but something felt off. A subsequent analysis comparing observations made in 2022 with data from NASA’s Neil Gehrels Swift Observatory pointed to another explanation: the tidal disruption of a massive gas cloud by the two black holes.

Anomalous Flares: Uncovering the Mystery

Unlike typical supermassive black holes that exhibit predictable activity, AT 2021hdr demonstrated a curious pattern by flaring every 60 to 90 days. Generally, the light emitted by active galactic nuclei (AGN) can vary due to different processes occurring near the black holes. In the case of AT 2021hdr, the flares were not powerful enough to indicate the presence of jets, which ruled out one possible source of variability.

Researchers observed striking increases in X-ray emissions, indicating flaring activity. However, typical AGN flares, known as quasi-periodic eruptions (QPEs), often show periods of dormancy. In contrast, AT 2021hdr remained persistently active without any noticeable dormancy, with flares visible in ultraviolet and optical wavelengths in addition to X-rays.

The Culprit: A Massive Gas Cloud?

After eliminating numerous potential causes, researchers honed in on the hypothesis that the flares were related to a gigantic gas cloud rather than a star. “In addition to stars, gas clouds can also be disrupted by supermassive black holes and their binaries,” said the team. The distinguishing factor is that gas clouds can be similar to or even larger than the distance between the black holes, a characteristic not applicable to stars.

Previous numerical simulations reinforced this viewpoint. As the black holes orbit each other, they are predicted to pull in significant amounts of material from the gas cloud—anywhere from three to 30 percent of the gas cloud during each orbit. Such substantial consumption implies that the gas cloud is extraordinarily large, providing a sustained source of feeding for the black holes.

A Glimpse into the Future

Forecasts suggest that the two supermassive black holes in AT 2021hdr will merge in approximately 70,000 years. They are also part of a larger cosmic dance as their host galaxy merges with a nearby galaxy—a fascinating layer to this ongoing celestial saga.

The continuing observations of AT 2021hdr could ultimately unveil more about its nature and whether it is indeed consuming a gas cloud. For the time being, it appears that these black holes have a taste for the very gas that surrounds them, offering an exciting glimpse into the mysteries of the universe.

Conclusion

Stay tuned, as deeper investigations will likely reveal more astonishing secrets about these celestial giants and their unusual behavior!