Astronomers detect most luminous transient event linked to black hole

The osbervation of J221951

This unusual event was discovered by chance while searching for the transient flare released by the merging of neutron stars — a kind of star nearing the end of its life. Such a merging event is called a kilonova. 

Astronomers at the University of Birmingham were looking for electromagnetic light emitted by gravitational waves via such mergers in September 2019. They utilized the Ultra-Violet and Optical Telescope on the Neil Gehrels Swift Observatory for this. 

As per the Royal Astronomical Society's official release, the hallmark of a kilonova is often blue. The transient gradually fades and turns reddish in hue over a period of days. 

However, in the case of J221951, this color shift did not occur. 

This prompted the team to perform more observations with telescopes such as NASA's Swift/UVOT and Hubble Space Telescopes, the South African Large Telescope, and ESO facilities such as the Very Large Telescope. 

The Hubble Space Telescope spectrum data ruled out the link of J221951 with the gravitational wave event.

A black hole located 10 billion light years away

J221951's light spectrum revealed that its source is around 10 billion light years away from us. J221951 has been identified as one of the most luminous transients ever recorded. 

The flare was tracked to the core of a galaxy and was caused by a supermassive black hole rapidly feasting on surrounding material after being quiet for a while. 

The researchers propose two probable causes for this black hole's rapid feeding behavior. 

First, it is likely released by a tidal disruption event, which occurs when a star passes from the vicinity of a supermassive black hole. The second hypothesis is that it is caused by an active galactic nucleus, which is likely 'changing status' from dormant to active.

"In the future, we will be able to obtain important clues that help distinguish between the tidal disruption event and active galactic nuclei scenarios. For instance, if J221951 is associated with an AGN turning on we may expect it to stop fading and to increase again in brightness, while if J221951 is a tidal disruption event we would expect it to continue to fade. We will need to continue to monitor J221951 over the next few months to years to capture its late-time behavior," concluded Dr. Samantha Oates, one of the study authors.

The findings were presented at the 2023 National Astronomy Meeting in Cardiff on Tuesday, July 4. The study will be published in Monthly Notices of the Royal Astronomical Society.