A pair of enormous black holes, together containing 40 million times the mass of our Sun, are churning up a cloud of gas at the centre of a distant galaxy.
NASA’s Neil Gehrels Swift Observatory, which is a space telescope designed to search for energetic bursts in the Universe, spotted the black hole pair.
More black hole stories
The black holes are in the centre of galaxy 2MASX J21240027+3409114, 1 billion lightyears away in constellation Cygnus.
They're separated by a distance of 16 billion miles (26 billion kilometres) and orbit each other every 130 days.
What's more, they're closing in on each other and will eventually collide in about 70,000 years' time.
Detecting the black hole pair
"It’s a very weird event, called AT 2021hdr, that keeps recurring every few months," says Lorena Hernández-García, astrophysicist at the Millennium Institute of Astrophysics, the Millennium Nucleus on Transversal Research and Technology to Explore Supermassive Black Holes, and University of Valparaíso in Chile.
"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."
AT 2021hdr was already known to astronomers, having been first detected in March 2021 by the Caltech-led ZTF (Zwicky Transient Facility) at the Palomar Observatory in California.
Astronomers noted it as a potentially interesting source, but couldn't quite work out what it was.
"Although this flare was originally thought to be a supernova, outbursts in 2022 made us think of other explanations," says 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 subsequent event has helped us refine our model of what’s going on in the system."
Further investigations needed...
Since that first flare, ZTF detected outbursts from AT 2021hdr every 60 to 90 days.
In 2022, Hernández-García and her team began observing the source with the Swift observatory, and were able to determine that the source was producing signals visible in ultraviolet and x-ray light on the same time scales as ZTF sees them in the optical light.
Initially, the prognosis was the signal could be a symptom of 'regular' activity in the centre of the galaxy.
Then the team thought they might be witnessing a 'tidal disruption event', which occurs when a star wanders too close and is ripped apart by a black hole (like the story of ASASSN-14li in 2023).
They've settled on the idea they're witnessing a gas cloud, bigger even than the black hole binary, being ripped apart.
As the gas cloud gets close to the black hole pair, the force of gravity tears it to pieces, causing filaments to fall in towards the black holes and heat up as they do so.
This gas becomes dense and hot and, as a result of gravitational interactions at play, some of the gas is ejected, causing the fluctuating light show.
And it's not only the black holes that are merging: the very galaxy in which they're in is also merging with another galaxy.
The team now plan to keep watching AT 2021hdr to get a better idea of what's going on.
A paper about AT 2021hdr, led by Hernández-García, was published 13 November 2024 in the journal Astronomy and Astrophysics.
