Scientists witnessed what could be a flare of light from a pair of colliding black holes, according to a study recently published in the journal Physical Review Letters.
If it's the real thing, we just witnessed two black holes merge in a cataclysmic explosion, shaking the fabric of space and time and sending calamitous gravitational waves out into the universe, in visible light, for the first time.
Black hole merger explodes with light
The merger was witnessed via the NSF's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector on May 21, 2019, in a wild event called S190521.
While the cosmic merger happened, Caltech's Zwicky Transient Facility (ZTF) was carrying out a robotic survey of the sky that seeks out cosmic flares, eruptions, or generally varying phenomena in the sky.
One night they caught a flare with origins in a distant active supermassive black hole — or quasar — called J1249+3449, located in the gravitational-wave event S100521g.
Correlating LIGO gravity waves to visual light signs
LIGO detects gravitational waves through laser interferometry — high-powered lasers measuring tiny changes in the distance between two objects, each placed kilometers apart, according to Ars Technica. On September 14, 5:51 AM EST, both detectors registered signals milliseconds apart for the first time — providing direct evidence of two black holes spiraling inward and into one another — merging in a cataclysmic collision event that sent powerful gravity shockwaves across the universe.
Since an event in 2017 called a "kilonova," astronomers have raced for a chance to see a corresponding optical signal when LIGO/VIRGO detected a gravitational wave signal for neutron star mergers or a possible neutron-star-black-hole merger. But the belief was that black hole on black hole collisions wouldn't leave an optical signature, which would mean there's no use in looking for flares.
However, last year an astronomer at Caltech and co-author of the paper proposed a different model — one that predicts a merger would give off an optical signature via intense flare, given a specific environment and conditions.
Black hole optical flares discovered
This is significant because Graham and others found the first possible evidence that their model could be right. In the case of this study, it correlates a binary black hole merger witnessed by LIGO on May 21, 2019 (event S190521g) with an intense flare, for the first time. The binary system could have formed in the accretion disk surrounding a supermassive black hole at the center of another galaxy.
The accretion disk is similar to a swarm of stars and dead stars — including black holes, according to co-author K.E. Saavik Ford of the City University of New York Graduate Center. "These objects swarm like angry bees around the monstrous queen bee at the center," she said. "They can briefly find gravitational partners and pair up but usually lose their partners quickly to the mad dance. But in a supermassive black hole's disk, the flowing gas converts the mosh pit of the swarm to a classical minuet, organizing the black holes so they can pair up."
When the binary pair inevitably merge, a new and larger black hole formed from the two experiences a powerful kick in gravitational force, and plows through the gas still lying in the accretion disk, which reacts with a bright flare — one that astronomers can see with a telescope.
For now, Graham and colleagues are performing a systemic search of their data that coincides with the extensive LIGO events detected so far from the collaboration, with aims to find more. This latest-formed black hole should create another energetic flare sometime in the next few year, when it re-enters the supermassive black hole's accretion disk once more.