Black hole collision may have exploded with light

An international team of astronomers, including researchers from the University of Minnesota, have seen what might amount to the first light ever detected from a black hole merger.

When two black holes spiral around each other and ultimately collide, they send out ripples in space and time called gravitational waves. Because black holes do not give off light, these events are not expected to shine with any light waves, or electromagnetic radiation. But some theorists have come up with ways in which a black hole merger might explode with light.

Now, for the first time, astronomers have seen evidence for one of these light-producing scenarios. The research is published in the journal Physical Review Letters, the American Physical Society’s flagship publication.

With the help of Caltech's Zwicky Transient Facility (ZTF), funded by the National Science Foundation (NSF) and located at Palomar Observatory near San Diego, the scientists have spotted what might be a flare of light from a pair of coalescing black holes. The black hole merger was first spotted by the NSF's Laser Interferometer Gravitational-wave Observatory (LIGO) and the European Virgo detector on May 21, 2019, in an event called S190521g. As the black holes merged, jiggling space and time, they sent out gravitational waves.

While this was happening, ZTF was performing its robotic survey of the sky that captured all kinds of objects that flare, erupt, or otherwise vary in the night sky. One flare the survey caught, generated by a distant active supermassive black hole, or quasar, called J1249+3449, was pinpointed to the region of the gravitational-wave event S190521g.

“One of the big questions in astrophysics is where black hole mergers generally come from,” said University of Minnesota physics and astronomy Assistant Professor Michael Coughlin, who is part of the ZTF team and a co-author of the publication. “‘Small’ black holes in the vicinity of ‘big’ black holes are one possibility, and the potential detection of an optical counterpart to a binary black hole merger in the vicinity of one of these supermassive black holes gives us a clue that this might be an important place these are produced.”

Coughlin, who helps lead the searches for gravitational-wave counterparts for binary neutron star and neutron star black holes with ZTF, said this detection also gives as a hint that while these binary black holes themselves do not emit light, they might disrupt their surroundings enough to make it possible to see their aftermath.

“This supermassive black hole was burbling along for years before this more abrupt flare,” said Matthew Graham, a research professor of astronomy at Caltech and lead author of the study. “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.”

How do two merging black holes erupt with light? In the scenario outlined by Graham and his colleagues, two partner black holes were nestled within a disk surrounding a much larger black hole.

“At the center of most galaxies lurks a supermassive black hole. It's surrounded by a swarm of stars and dead stars, including black holes,” said co-author K. E. Saavik Ford of the City University of New York (CUNY) Graduate Center, the Borough of Manhattan Community College (BMCC), and the American Museum of Natural History (AMNH). “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,” she said.

Once the black holes merge, the new, now-larger black hole experiences a kick that sends it off in a random direction, and it plows through the gas in the disk.

“It is the reaction of the gas to this speeding bullet that creates a bright flare, visible with telescopes,” said co-author Barry McKernan, also of the CUNY Graduate Center, BMCC, and AMNH. 

Such a flare is predicted to begin days to weeks after the initial splash of gravitational waves produced during the merger. In this case, ZTF did not catch the event right away, but when the scientists went back and looked through archival ZTF images months later, they found a signal that started days after the May 2019 gravitational-wave event. ZTF observed the flare slowly fade over the period of a month.

The scientists attempted to get a more detailed look at the light of the supermassive black hole, called a spectrum, but by the time they looked, the flare had already faded. A spectrum would have offered more support for the idea that the flare came from merging black holes within the disk of the supermassive black hole. However, the researchers say they were able to largely rule out other possible causes for the observed flare, including a supernova or a tidal disruption event, which occurs when a black hole essentially eats a star.

What is more, the team says it is not likely that the flare came from the usual rumblings of the supermassive black hole, which regularly feeds off its surrounding disk. Using the Catalina Real-Time Transient Survey, led by Caltech, they were able to assess the behavior of the black hole over the past 15 years, and found that its activity was relatively normal until May of 2019, when it suddenly intensified.

“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, an assistant professor of astronomy at Caltech. “The reason looking for flares like this is so important is that it helps enormously with astrophysics and cosmology questions. If we can do this again and detect light from the mergers of other black holes, then we can nail down the homes of these black holes and learn more about their origins.”

The newly formed black hole should cause another flare in the next few years. The process of merging gave the object a kick that should cause it to enter the supermassive black hole's disk again, producing another flash of light that ZTF should be able to see.

The research was funded by the NSF, NASA, the Heising-Simons Foundation, and the GROWTH (Global Relay of Observatories Watching Transients Happen) program. ZTF is also funded by an international collaboration of partners, with additional support from NASA, the Heising-Simons Foundation, members of the Space Innovation Council at Caltech, and Caltech itself. 

To read the full research paper entitled “Candidate Electromagnetic Counterpart to the Binary Black Hole Merger Gravitational Wave Event GW190521g,” visit the Physical Review Letters website.