Galaxy 2MASX J21240027+340911 has been known to have an active nucleus for a decade or so. At its core, a supermassive black hole was feeding on some interstellar material, gas or dust, that got too close. Recently, astronomers have discovered a repeating signal from this object, suggesting more complexity: It is not one but two supermassive black holes in the core of this galaxy – and they are sharing a meal.
The supermassive black holes have a combined mass 40 million times that of the Sun and they are about one light-day away from each other, roughly 26 billion kilometers (16 billion miles). They are destined to collide in about 70,000 years, and they orbit each other ever more closely every 130 days. It is this orbital motion that gave rise to the repeating event observed.
“It’s a very weird event, called AT 2021hdr, that keeps recurring every few months,” lead author Lorena Hernández-García, an astrophysicist at the Millennium Institute of Astrophysics and University of Valparaíso in Chile, said in a statement. “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.”
Artist’s impression of the pair of monster black holes swirling in a cloud of gas
NASA/Aurore Simonnet (Sonoma State University)
The outburst was first seen by the Caltech-led ZTF (Zwicky Transient Facility) at the Palomar Observatory. The facility continued to spot the event every 60 to 90 days, and it was then followed up by NASA’s Swift telescope.
“Although this flare was originally thought to be a supernova, outbursts in 2022 made us think of other explanations,” added co-author Alejandra Muñoz-Arancibia, also 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.”
The team considered multiple possibilities. It could have been a common behavior in an active nucleus. Alternatively, it might have been a star getting too close to the supermassive black hole, getting ripped apart, and being slowly consumed. But a pair of supermassive black holes shrouded in a gas cloud, having a feast while orbiting each other, is the most compelling.
The team plans to continue monitoring this event to better model what is going on, as well as studying the host galaxy which is undergoing a merger. The galaxy is 1 billion light-years from Earth.
The paper is published in the journal Astronomy & Astrophysics.
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