A pair of monster black holes swirl in a cloud of gas in this artist’s concept of AT 2021hdr. Credit: NASA/Aurore Simonnet
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 . In 2022, another outburst was observed, and over time the Zwicky Transient Facility found a pattern of outbursts every 60–90 days.
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.