Two Black Holes Colliding Not Enough? Make It Three

In 2016, scientists for the first time detected the collision of two distant black holes, using the Laser Interferometer Gravitational-Wave Observatory, or LIGO, a pair of L-shaped antennas in Hanford, Wash., and Livingston, La. Since then LIGO and a third antenna, Virgo, located in Italy, together have charted dozens of similar catastrophic marriages out there in the dark. But astronomers have yet to see any trace of light from them. (One exception was a collision of neutron stars, the remnants of supernova explosions, that lit up the universe and was detected in August 2017)

On May 21, 2019, an alert went out to the world’s astronomers that the LIGO and Virgo antennas had recorded what looked like two black holes colliding. Among the telescopes on duty that night was the Zwicky Transient Facility, a robotic instrument on Palomar Mountain in California, which monitors the deep sky for anything that flares, blinks, explodes or moves. It is named after Fritz Zwicky, an innovative and eccentric Swiss astronomer who worked at Caltech.

Dr. Graham, the project scientist for the Zwicky telescope, and his colleagues had been mulling the possibility that black hole mergers might be happening in the dense, sparky accretion disks of supermassive black holes, which are the central engines for quasars. The team began monitoring quasars in the those regions for unusual activity.

The trail from the May gravitational wave event led to a quasar known as J124942.3+344929, located about 4 billion light years from Earth. Examining records from the Zwicky telescope, Dr. Graham discovered that the quasar had flared, doubling in brightness for about a month — an uncharacteristically large fluctuation. That marked it as a possible black hole collision, he said.

Bolstering that hypothesis was the fact that the flare did not become visible until 34 days after the gravitational waves were detected. It would take about that long for any light from a black hole collision to emerge from such a thick disk of gas, according to a model that Dr. Ford and Barry McKernan, her colleague at the American Museum of Natural History, described in a paper last year.

Dr. Ford described the accretion disk as “ a swarm of stars and dead stars, including black holes,” in a Caltech news release.

She added, “These objects swarm like angry bees around the monstrous queen bee at the center. 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.”