Monster black holes are trapped in an eternal dance

Black holes lie at the heart of an extraordinary cosmic event that is currently unfolding. With the help of NASA’s Neil Gehrels Swift Observatory, scientists have detected a peculiar phenomenon involving a pair of what they refer to as “monster” black holes.

“It’s a very weird event, called AT 2021hdr, that keeps recurring every few months,” explained Lorena Hernández-García, an astrophysicist at 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.

This cosmic spectacle is believed to involve the two monstrous black holes engulfing an unsuspecting gas cloud. As these massive bodies orbit each other, their gravitational forces interact with the cloud, stirring and consuming its gas.

This process creates an oscillating pattern of light emissions that scientists have been observing for months during the dramatic and recurring cosmic performances.

The dance of the monster black holes

The monster black holes reside deep within a galaxy known as 2MASX J21240027+3409114. This celestial neighborhood is nestled about 1 billion light-years away in the northern constellation Cygnus.

Trapped in their eternal celestial dance, the black holes are separated by around 16 billion miles, a distance close enough for light to travel between them within a day.

These cosmic giants together have a mass 40 million times that of our own sun. The researchers have determined that these black holes complete a single orbit every 130 days.

Looking into the far future, by about 70,000 years to be precise, the scientists predict a grand finale when these twin behemoths finally collide and merge.

Mysterious recurring outbursts

The journey to uncovering AT 2021hdr began in March 2021, when the Caltech-led ZTF (Zwicky Transient Facility) at the Palomar Observatory in California identified it as an unusual source.

The event was quickly flagged by ALeRCE (Automatic Learning for the Rapid Classification of Events), a collaborative team that integrates artificial intelligence with human analysis to sift through extensive survey data and highlight significant night sky phenomena for the astronomical community.

With initial ideas attributing the flare to a supernova, the recurring nature of the outbursts prompted a shift in hypotheses. Since the first flare, the event has consistently produced outbursts, roughly every 60 to 90 days.

Study co-author Alejandra Muñoz-Arancibia is an ALeRCE team member and astrophysicist at the Millennium Institute of Astrophysics and the Center for Mathematical Modeling at the University of Chile.

“Although this flare was originally thought to be a supernova, outbursts in 2022 made us think of other explanations,” said Muñoz-Arancibia. “Each subsequent event has helped us refine our model of what’s going on in the system.”

Monster black holes devour a gas cloud

The researchers left no stone unturned in their quest to unravel this cosmic mystery. Their efforts rewarded them with a revelation: the binary system produces oscillations in ultraviolet and X-ray light on the same time scales as ZTF observes in the visible range.

Through a process of elimination, the experts first dismissed the possibility of regular activity in the galactic center being the cause. Next, they eliminated the chance of a star wandering too close to the black holes and being ripped apart.

Eventually, the scientists settled on the theory of a gas cloud, much larger than the binary itself, meeting its fate at the hands of the twin black holes. As the cloud approached the black holes, gravity tore it apart, forming filaments around the pair, and the ensuing friction started to heat it.

As the gas cloud got denser and hotter near the black holes, the complex interplay of forces spewed out some of the gas with each rotation – causing the fluctuations in light that Swift and ZTF have been observing.

Unveiling the grandeur of the universe

Looking ahead, the scientists plan to continue their observations of AT 2021hdr. They anticipate further insights into the system and aim to hone their models.

In particular, the scientists are interested in studying the host galaxy of AT 2021hdr, which is currently merging with another nearby galaxy – a phenomenon that they were the first to report.

Reflecting on the mission, S. Bradley Cenko, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, expresses his pride and awe at the discoveries that Swift continues to enable as it approaches its 20th anniversary.

“There’s still so much it has left to teach us about our ever-changing cosmos,” said Cenko.

Through this venture, NASA continues its commitment to unveiling the grandeur of our universe and deciphering its many mysteries.

Its missions are an integral part of an expanding, global network that observes the sky unceasingly to unravel how the universe works.

The study is published in the journal Astronomy and Astrophysics.

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