The Enigmatic Titans: Black Holes that Devour Gas in Cosmic Feasts

“This event is very unusual and known as AT 2021hdr, and it happens repeatedly every few months,” explained Lorena Hernández-García, an astrophysicist from the Millennium Institute of Astrophysics, the Millennium Nucleus for Transversal Research and Technology to Investigate Supermassive Black Holes, and the University of Valparaíso in Chile. “We suspect that a gas cloud has captured the black holes. While the black holes orbit each other, they interact with the cloud, agitating and consuming its gas. This interaction causes a fluctuating light pattern from the system.”

A research paper about AT 2021hdr, led by Hernández-García, was published on November 13 in the journal Astronomy and Astrophysics.

These twin black holes reside in a galaxy known as 2MASX J21240027+3409114, which is situated 1 billion light-years away in the northern constellation of Cygnus. The two black holes are approximately 16 billion miles (or 26 billion kilometers) apart, a distance that light can cover in just one day. Collectively, they possess a mass equivalent to 40 million suns.

Researchers believe the black holes complete an orbit every 130 days and are expected to merge in about 70,000 years.

The phenomenon AT 2021hdr was initially observed in March 2021 by the Zwicky Transient Facility (ZTF), a project led by Caltech located at the Palomar Observatory in California. It was highlighted as a potentially notable source by ALeRCE (Automatic Learning for the Rapid Classification of Events). This diverse team uses a blend of artificial intelligence and human analysis to inform the astronomical community about notable events from the vast amounts of data collected by survey initiatives like ZTF.

“At first, we thought this flare was a supernova, but more outbursts in 2022 led us to consider other possibilities,” noted 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 is happening in this system.”

Since the initial flare, ZTF has identified outbursts from AT 2021hdr occurring every 60 to 90 days.

Starting in November 2022, Hernández-García and her team began tracking this source with Swift. This observatory enabled them to confirm that the binary system produces oscillations in ultraviolet and X-ray light that coincide with the fluctuations observed in visible light by ZTF.

The researchers methodically eliminated various models to explain their findings.

Initially, they thought the signal might just be a result of regular activities at the galactic center. Then they considered the possibility of a tidal disruption event, which occurs when a star strays too close to one of the black holes.

Ultimately, they concluded that a different scenario was more likely: the tidal disruption of a gas cloud larger than the binary itself. As this cloud interacts with the black holes, gravitational forces pull it apart, creating filaments around the black holes, while friction heats the gas. The vicinity near the black holes sees a significant increase in gas density and temperature. As the black holes orbit, their gravitational interplay ejects some gas from the system with each rotation, which results in the varying light patterns observed by Swift and ZTF.

Hernández-García and her team intend to keep monitoring AT 2021hdr to further comprehend the system and enhance their models. They are also keen on examining its host galaxy, which is currently in the process of merging with a nearby galaxy, a finding first highlighted in their paper.

“As we approach Swift’s 20th anniversary, it’s amazing to witness the new discoveries it continues to support,” remarked S. Bradley Cenko, Swift’s principal investigator at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “There is still a wealth of knowledge it holds regarding our ever-evolving universe.”