Two Supermassive Black Holes on the Verge of a Merger

In March 2021, astronomers observed a high-energy burst of light from a distant galaxy. Assigned the name AT 2021hdr, it was thought to be a supernova. However, there were enough interesting features that flagged as potentially interesting by the Automatic Learning for the Rapid Classification of Events (ALeRCE). In 2022, another outburst was observed, and over time the Zwicky Transient Facility (ZTF) found a pattern of outbursts every 60–90 days. It clearly wasn’t a supernova, but it was unclear on what it could be until a recent study solved the mystery.

One idea was that AT 2021hdr was a tidal disruption event (TDE),] where a star strays too close to a black hole and is ripped apart. This can create periodic bursts as the stellar remnant orbits the black hole, but TDEs don’t tend to have such regular patterns. So the team considered another model, where a massive interstellar cloud passes into the realm of a pair of binary black holes.

Simulations show how binary black holes interact with a gas cloud. Credit: F. Goicovic et al. 2016

Computer simulations show that rather than simply ripping apart the cloud, a binary black hole would churn the cloud as it consumes it. This would produce a periodic burst of light as the black holes orbit. The team observed AT 2021hdr using the Neil Gehrels Swift Observatory and found periodic oscillations of ultraviolet and X-ray light that match the transient bursts observed by ZTF. These observations match the simulations of a binary black hole.

Based on the data, the black holes have a combined mass of about 40 million Suns, and they orbit each other every 130 days. If they continue along their paths, the two black holes will merge in about 70,000 years. Without the passing cloud, we would have never noticed them.

The team plans to continue their observations of the system to further refine their model. They also plan to study how the black holes interact with their home galaxy.

Reference: L. Hernández-García, et al. “AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system.” Astronomy & Astrophysics 691 (2024)

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