Astronomers May Have Observed The First Flare From A Black Hole Collision

Artist's concept of a supermassive black hole and its surrounding disk of gas. Embedded within this disk are two smaller black holes about to merge with each other. Caltech/R. Hurt (IPAC)

For the first time, astronomers may have observed a light signature for a collision between black holes. The black hole merger was detected by the LIGO and Virgo gravitational wave observatories. The research is published in Physical Review Letters.
 
Whenever a gravitational wave signal is detected, an alert is sent out to telescopes and observatories across the world to try and spot a light signature related to the event. Only one signature has been discovered before this event, and it was related to the first known collision between neutron stars. The most recent event, known as S190521g, involved two stellar-sized black holes as well as a third supermassive black hole.
 

Supermassive black holes can be surrounded by a large disk of material and orbited by stars as well as black holes. The team believes that a peculiar black hole three-way has happened, whereby two blacks holes are merging in the disk surrounding the supermassive black hole. The collision sent the newly formed black hole on a new trajectory, pummeling through the disk and disrupting the material with its gravity, producing a flare. The black hole produced by the merger had a total mass 100 times that of the Sun and moved through the disk at 200 kilometers (125 miles) per second.

The suggestion of this unorthodox scenario comes from observations by Caltech’s Zwicky Transient Facility (ZTF), which performs a robotic survey of the sky. Once the gravitational event was discovered, an alert was sent out to telescopes across the world, but they did not spot anything. A few days later, the ZTF recorded a flare from a supermassive black hole, which was roughly in the right part of the sky and at a distance expected for the gravitational wave event. The team only became aware of this months later while looking through archival data. By that point, the flare had already faded and they couldn't confirm its actual source.

“This supermassive black hole was burbling along for years before this more abrupt flare,” lead author Matthew Graham, a research professor of astronomy at Caltech and the project scientist for ZTF, said in a statement. “The flare occurred on the right timescale, and in the right location, to be coincident with the gravitational-wave event. In our study, we conclude that the flare is likely the result of a black hole merger, but we cannot completely rule out other possibilities.”  

The team believe they might get a chance to confirm whether or not their scenario is indeed correct in a few years. The newly formed black will now be on a trajectory that will take it back into the disk as it orbits the supermassive black hole. If a similar flare happens, they should be able to tell what caused it. 

This is an excellent week for gravitational wave astronomy. LIGO and Virgo have just announced the discovery of what is either the heaviest neutron star or the lightest black hole.

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