Colliding black holes the size of Lucknow merged to create light for the first time

• In a first for the universe, two black holes may have merged to create a flare of light.
  
• The secret to this astronomical event is the fact that they were circling a third supermassive black hole with the mass of millions of suns.
  
• Another flare may be in the works over the next few years, as the kick from the black hole merger should cause it to enter the supermassive black hole’s disk again.
  

A black hole merger between two partner black holes — with a combined mass of 150 suns but only as big as Lucknow — may have exploded in a flare of light for the first time ever.

Normally when black holes merge they produce gravitational waves that ripple out across the universe, but they have never been known to give off any light. However, the possibility has long been speculated by scientists.

This event — called S190521g — was spotted by Caltech's Zwicky Transient Facility (ZTF). Described in the Physical Review Letters the burst of light happened because the two black holes were orbiting a third, must larger, supermassive black hole.

Creating light out of darkness when two black holes collide
"These objects swarm like angry bees around the monstrous queen bee at the centre,” said co-author KE Saavik Ford. She explains that in most cases black holes find their gravitational partners but usually drift apart after a while in all the madness. In the case of a supermassive black hole, the flowing gas flows into a kind of ballroom dance, organising the smaller black holes to remain in a synchronised dance around each other.

When the black holes finally merge — which a more violent in the vast expanse of space — the resulting kick ploughs through the gas like a speeding bullet that creates a bright flare, visible even with telescopes.

The flash is difficult to spot because it’s predicted to begin days, or even weeks, after the initial burst of gravitational waves due to the merger of the black holes.

In the case of S190521g, ZTF didn’t catch the event right way when it occurred in May 2019. It was only when a team of scientists went back to look through archived data, they found the signal, which started days after the initial event.

Other explanations may also exist
The team at ZTF observed the flare slowly fade over the course of one month. They tried to get a more detailed look at the light of the supermassive black hole — called a spectrum — but by the time they spotted it, it had already faded quite a bit.

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," said Matthew Graham, the lead author of the study. However, the study points out that the scientists were largely able to rule other explanations like a supernova or a tidal disruption event — when a black hole eats a star.

Their observations show that the behaviour of the black hole was fairly constant over the last 15 years. It was only in May when there was a sudden surge."Supermassive black holes like this one have flares all the time. They are not quiet objects, but the timing, size, and location of this flare was spectacular," said co-author Mansi Kasliwal.

In the next few years, the new black hole is likely to cause another flare since the kick from the merger should cause it to enter the supermassive black hole’s disk again.

In order to confirm the findings, Ligo scientists are likely to publish their own observations in the coming months.

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