Ancient Rome was teetering. Then a volcano erupted 6,000 miles away.

By Dennis Overbye

In an announcement last week, astronomers described the detection of an epistemological marvel: an invisible collision of invisible objects — black holes — had become briefly visible. The story goes like this:

Long, long ago, about 4 billion years before now and in a faraway galaxy, a pair of black holes collided. Typically such an event would leave no visible trace, just a shuddering of space-time — gravitational waves — and a bigger black hole. (Black holes emit no light.) But these black holes were part of a swirl of star parts, gas and dust surrounding a third, gigantic black hole, a supermassive black hole 100 million times more massive than the sun. As a result, the merging pair generated a shock wave of heat and light that allowed the collision to be seen as well as heard.

That is the explanation being offered by a group of astronomers, led by Matthew Graham of the California Institute of Technology, for a curious flash of light they recorded last year. Their conclusion, announced Thursday, was laid out in a paper in Physical Review Letters.

If the result holds up, it would mark the first time that colliding black holes have produced light as well as gravitational waves. “We have seen a visible signal from a previously invisible part of the universe,” Graham said.

K.E. Saavik Ford, of the American Museum of Natural History and the City University of New York and an author of the new study, said about the black holes, “It means we can see them and hear them at the same time.” She called the whole event “super exciting.”

The work, the researchers say, could lead to new insights into how, when and where black holes merge into ever bigger monsters that weigh millions or billions of suns and dominate the centers of galaxies. It could also elucidate the conditions inside the crackling turnstile of fire and fury through which matter passes on its way to black-hole doom.

Two black holes colliding while in the whirling grip of another? “Astrophysics probably doesn’t get more exciting than that,” Graham said.

Black holes are objects predicted by Albert Einstein to be so dense that not even light can escape them. Most of the black holes that astronomers know about are the corpses of massive stars that have died and collapsed catastrophically into nothing; the dark remnants are a few times as massive as the sun. But galaxies harbor black holes millions or billions of more massive than that. How black holes can grow so big is an abiding mystery of astronomy.

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-shape antennas in Hanford, Washington, and Livingston, Louisiana. Since then LIGO and a third antenna, Virgo, 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.

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, 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 had been 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 Ford and Barry McKernan, her colleague at the American Museum of Natural History, described in a paper last year.

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.”

The result, she said, can be a frenzy of black holes combining and recombining into bigger and bigger cosmic graves. This, she said, is what might have caused the signal that was detected in May 2019.

That could explain how the black holes in this collision grew so big, she said. The black hole that emerged from this collision and left a fiery trail through the accretion disk was at least 100 times as massive as the sun. But 50 solar masses is the weight limit for black holes formed directly from dead stars, meaning that the two holes that collided in May 2019 were right at the limit and probably even bigger. So they didn’t result directly from a stellar collapse, she said. Rather, they probably formed through a series of ever-larger mergers.

The collision heard by LIGO and Virgo might have been only the end of a chain reaction of black holes mating. “This is the tip of the iceberg,” Ford said.

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