Reviewed by Lexie CornerNov 14 2024
Scientists have detected the signal from two giant black holes interacting with a cloud of gas in the center of a galaxy for the first time, using images from NASA’s Neil Gehrels Swift Observatory, according to a study published in Astronomy and Astrophysics.
A pair of monster black holes swirl in a cloud of gas in this artist’s concept of AT 2021hdr, a recurring outburst studied by NASA’s Neil Gehrels Swift Observatory and the Zwicky Transient Facility at Palomar Observatory in California. Image Credit: NASA/Aurore Simonnet (Sonoma State University)
It is a very weird event, called AT 2021hdr, that keeps recurring every few months. We think that a gas cloud engulfed the black holes. As they orbit each other, the black holes interact with the cloud, perturbing and consuming its gas. This produces an oscillating pattern in the light from the system.
Lorena Hernández-García, Astrophysicist, Millennium Institute of Astrophysics, University of Valparaíso
The galaxy 2MASX J21240027+3409114, located in the northern constellation Cygnus, is situated 1 billion light-years away from the pair of black holes. The two black holes are separated by approximately 16 billion miles (26 billion kilometers), and light takes about a day to travel between them. Their combined mass is 40 million times that of the Sun.
Scientists believe the black holes complete an orbit every 130 days and are expected to merge in about 70,000 years.
The event AT 2021hdr was first detected in March 2021 by the Caltech-led Zwicky Transient Facility (ZTF) at Palomar Observatory in California. It was identified as a potentially significant source by ALeRCE, a multidisciplinary team that uses artificial intelligence algorithms and human expertise to track and report night sky phenomena, analyzing large datasets from survey programs like ZTF.
Although this flare was originally thought to be a supernova, outbursts in 2022 made us think of other explanations. Each subsequent event has helped us refine our model of what’s going on in the system.
Alejandra Muñoz-Arancibia, Study Co-Author and Astrophysicist, University of Chile
Since the first flare, ZTF has detected outbursts from AT 2021hdr approximately every 60-90 days.
Hernández-García and her team have been monitoring the source with Swift since November 2022. Swift helped them confirm that the binary system produces oscillations in ultraviolet and X-ray radiation on the same time scales observed by ZTF in the visible spectrum.
To interpret the data, the researchers ruled out several possible explanations. Initially, they considered the signal might be due to typical activity in the galactic center. They then explored the idea that a tidal disruption event—a star being destroyed after getting too close to one of the black holes—could be responsible.
Ultimately, they concluded that the signal likely results from the tidal disruption of a gas cloud larger than the binary system itself. As the cloud collided with the black holes, gravity tore it apart, forming filaments around the pair. Friction from these interactions heated the gas, particularly near the black holes. As the black holes orbited, the forces involved expelled parts of the gas with each rotation, producing the flickering light detected by Swift and ZTF.
Hernández-García and her team plan to continue monitoring AT 2021hdr to refine their understanding of the system and improve their models. They are also interested in studying the galaxy hosting the system, which is currently merging with a nearby galaxy—an event originally noted in their research.
As Swift approaches its 20th anniversary, it is incredible to see all the new science it is still helping the community accomplish. There is still so much it has left to teach us about our ever-changing cosmos.
S. Bradley Cenko, Principal Investigator, Goddard Space Flight, National Aeronautics and Space Administration
NASA’s missions are part of an expanding global network focused on detecting changes in the sky to address questions about the workings of the universe.
Goddard leads the Swift project in collaboration with Penn State, the Los Alamos National Laboratory in New Mexico, and Northrop Grumman Space Systems in Dulles, Virginia. Additional partners include the University of Leicester and Mullard Space Science Laboratory in the United Kingdom, the Brera Observatory in Italy, and the Italian Space Agency.
Journal Reference:
Hernández-García, L., et al. (2024) AT 2021hdr: A candidate tidal disruption of a gas cloud by a binary super massive black hole system. Astronomy & Astrophysics. doi.org/10.1051/0004-6361/202451305.