Researchers have identified a pair of black holes located in the galaxy 2MASX J21240027+3409114, approximately one billion light-years away in the constellation Cygnus. The two black holes are separated by about 26 billion kilometers, allowing light to travel between them in just one day. Together, they possess a mass equivalent to 40 million times that of the Sun.
Estimates suggest that these black holes complete an orbit every 130 days and are on a collision course, expected to merge in roughly 70,000 years. The system, designated AT 2021hdr, was first detected in March 2021 by the Zwicky Transient Facility (ZTF) at Caltech's Palomar Observatory in California. It was flagged as a potentially interesting source by the Automatic Learning for the Rapid Classification of Events (ALeRCE) team.
This multidisciplinary team employs artificial intelligence alongside human expertise to communicate celestial events to the astronomical community, utilizing vast data collected by survey programs like ZTF.
Since its initial detection, ZTF has observed AT 2021hdr exhibiting explosions every 60 to 90 days. Hernández-García and his team have been monitoring the source with the Swift satellite since November 2022. Swift's observations revealed that this binary system produces oscillations in ultraviolet and X-ray light that correspond to the visible light variations detected by ZTF.
Initially, researchers speculated that the signal could be a byproduct of normal galactic activity. They later considered a tidal disruption event, where a star is destroyed by one of the black holes, as a possible cause. Ultimately, they concluded that the signal originates from the tidal disruption of a gas cloud larger than the binary itself. When the cloud approached the black holes, gravitational forces tore it apart, creating filaments around the pair and heating the gas through friction. The gas becomes particularly dense and hot near the black holes.
As the binary orbits, the complex interplay of forces ejects some gas from the system with each rotation, producing the fluctuating light observed by Swift and ZTF.
Hernández-García and his team plan to continue their observations of AT 2021hdr to gain a deeper understanding of the system and refine their models. They are also interested in studying the host galaxy, which is currently merging with another nearby galaxy, a phenomenon first reported in their article.