Researchers leveraging data from NASA’s Neil Gehrels Swift Observatory have identified, for the first time, a signal emanating from two massive black holes as they disrupt a gas cloud at the heart of a galaxy.
“It’s a highly unusual event, named AT 2021hdr, that reoccurs every few months,” stated Lorena Hernández-García, an astrophysicist affiliated with the Millennium Institute of Astrophysics, the Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes, and the University of Valparaíso in Chile. “We believe a gas cloud has enveloped the black holes. As these black holes revolve around each other, they engage with the cloud, disturbing and consuming its gas. This results in a fluctuating pattern in the light emitted from the system.”
A detailed study on AT 2021hdr, spearheaded by Hernández-García, was published on November 13 in the journal Astronomy and Astrophysics.
The two black holes are situated at the core of a galaxy identified as 2MASX J21240027+3409114, which lies 1 billion light-years away in the northern constellation Cygnus. The black holes are approximately 16 billion miles (26 billion kilometers) apart, a distance so close that light travels between them in just one day. Combined, their mass is about 40 million times that of the Sun.
Scientists estimate that the black holes complete one orbit around each other every 130 days and are expected to collide and merge in roughly 70,000 years.
AT 2021hdr was initially detected in March 2021 by the Zwicky Transient Facility (ZTF) led by Caltech at the Palomar Observatory in California. It was marked as a potentially intriguing source by ALeRCE (Automatic Learning for the Rapid Classification of Events). This interdisciplinary team uses artificial intelligence along with human expertise to report astronomical events using the extensive data gathered by survey programs like ZTF.
“Initially, this flare was thought to be a supernova, but subsequent outbursts in 2022 made us consider alternative explanations,” explained co-author Alejandra Muñoz-Arancibia, an ALeRCE team member and astrophysicist at the Millennium Institute of Astrophysics and the Center for Mathematical Modeling at the University of Chile. “Each new event has allowed us to refine our understanding of what’s happening in the system.”
Since the initial flare, ZTF has observed outbursts from AT 2021hdr every 60 to 90 days.
Hernández-García and her team have been monitoring the source with Swift since November 2022. Swift’s observations revealed that the binary system produces oscillations in ultraviolet and X-ray light on the same time scales as those observed by ZTF in the visible spectrum.
The researchers systematically ruled out various models to explain their observations.
Initially, they considered that the signal could be the result of regular activity in the galactic center. They also pondered whether a tidal disruption event—where a star is destroyed after straying too close to one of the black holes—could be the cause.
Eventually, they concluded that the most likely scenario was the disruption of a gas cloud, one larger than the binary system itself. When the cloud encountered the black holes, their gravitational forces tore it apart, creating filaments around the pair and causing friction that heated the gas. The gas became especially dense and hot near the black holes. As the binary system orbits, the complex interplay of forces ejects some of the gas with each rotation. These interactions produce the fluctuating light observed by Swift and ZTF.
Hernández-García and her team intend to continue their observations of AT 2021hdr to gain a deeper understanding of the system and refine their models. They are also keen on studying the galaxy where the event is taking place, which is currently merging with a neighboring galaxy—an occurrence first reported in their study.
“As Swift approaches its 20th anniversary, it’s amazing to witness the new scientific discoveries it continues to enable,” remarked S. Bradley Cenko, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “There is still so much more it can reveal about our ever-changing universe.”
NASA’s missions are part of an expanding, global network dedicated to observing changes in the sky to solve the mysteries of how the universe operates.
The Swift mission is managed by Goddard 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, Brera Observatory in Italy, and the Italian Space Agency.
Source: NASA
Original text: By Jeanette Kazmierczak
NASA’s Goddard Space Flight Center, Greenbelt, Md.