More than four billion years ago a pair of black holes engaged in a dramatic dance. With gas and dust whirling around them, the voids spiraled closer and closer together, orbiting perilously near to a supermassive black hole. Then the two smaller black holes merged in a collision so powerful that it shook the fabric of space itself. As the resulting gravitational waves rippled toward Earth, the merged black hole recoiled like a rocket, heating the surrounding gas and generating a blinding flare of light that lasted for weeks.
That scenario was proposed in a paper published in June 2020 in Physical Review Letters. What researchers know for sure is that on May 21, 2019, they detected gravitational waves emitted by a collision of two black holes. And in the following weeks a strange flare of light emerged from the same region of the sky. If the flare did indeed come from the black holes, it marks the first time that scientists have seen light emitted in the aftermath of a black hole merger.
“I was stunned,” says Jillian Bellovary, an assistant professor at Queensborough Community College, City University of New York, who was not involved in the work. “The idea is amazing. It’s jaw-droppingly amazing.” If confirmed, the finding offers a new direction for the fledgling field of multimessenger astronomy. By studying black holes and their surroundings with both gravitational waves and electromagnetic radiation, scientists expect to learn more than they could with either signal alone.
Scientists have long debated whether a binary black hole merger could trigger a flare of light in addition to gravitational waves. Most researchers agree that black holes colliding in a vacuum cannot produce light. But an active galactic nucleus, where the gravity of a supermassive black hole draws gas and dust into an immense accretion disk, would offer the perfect conditions to create a flare. “It’s a whole new way of making black holes merge,” says Chiara Mingarelli, who studies the celestial objects at the Center for Computational Astrophysics at the Flatiron Institute in New York City and was not involved with the research.
With this possibility in mind, researchers turned to the Zwicky Transient Facility (ZTF). Based at the Palomar Observatory in California, the facility photographs the entire northern sky every three nights and can spot any sudden changes in brightness. The team combed ZTF data in search of flares of light emanating from active galactic nuclei near black hole mergers that were previously detected by the Laser Interferometer Gravitational-wave Observatory (LIGO). In almost every case, the scientists found nothing. But in the aftermath of the May 2019 merger, an active galactic nucleus in the same region of the sky doubled in brightness. The researchers evaluated all the possible sources of the flare that they could think of and concluded that its most likely source was the merged black hole.
Still, they caution that they cannot be certain that their explanation is correct. And despite some scientists’ enthusiasm, other experts are highly skeptical of the researchers’ conclusion. “I just cannot understand how they would write the paper with that interpretation,” says Avi Loeb, a theoretical physicist and the founding director of Harvard University’s Black Hole Initiative. “It makes no sense.”
A Merger Mystery
Last summer’s announcement was not the first time astronomers thought they had seen light emitted by merging black holes. In 2015 the Fermi Gamma-ray Space Telescope spotted a faint burst of gamma rays coming from the same area as the very first black hole merger detected by LIGO. The signal sparked a flurry of speculation and competing explanations. Soon after, Loeb proposed that the pair of black holes had merged inside a star, which then provided the gas needed to emit the light. The research community remained unconvinced. Many scientists eventually conceded that the gamma-ray burst was likely a fluke—an emission from some separate background object that was completely unrelated to the black hole merger.
Loeb still thinks that under the right conditions, merging black holes could trigger a flare of light. But “I have a big problem with the suggestion that this flare was a result of accretion onto the [merged] black hole,” he says. The brightness of astronomical objects typically has an upper limit, known as the Eddington luminosity. In some circumstances, this limit can be exceeded by a factor of up to several thousand. In the scenario proposed in the study, however, the merged black hole had a luminosity around 100,000 times the Eddington value. “That is unheard of,” Loeb says.
Matthew Graham, project scientist at ZTF and lead author of the new paper, acknowledges that a luminosity so large would be unprecedented. But based on current knowledge, “it’s not unrealistic. It’s not beyond the bounds of possibility,” he says. Although running reliable computer simulations of the complex interactions of binary black holes in an active galactic nucleus is not yet feasible, extrapolation from past simulations convinced Graham’s team that its interpretation of the data was the most plausible.
“Reasonable people can disagree about it, but we can have a discussion,” says Barry McKernan, a co-author of the paper and an astrophysicist at Borough of Manhattan Community College, C.U.N.Y. If the flare of light did not originate from merging black holes, it might have come from a “very, very unusual supernova,” says study co-author K. E. Saavik Ford, also at Borough of Manhattan Community College. Another possible source is the natural variability of the active galactic nucleus, which is known to change in brightness over time. Loeb suspects the former scenario, whereas Zoltan Haiman of Columbia University favors the latter. Given the intensity and timing of the flare, both explanations seem highly unlikely to Ford and McKernan—though not impossible.
Unlike the gamma-ray burst in 2015, the new flare allowed the researchers to make predictions that will soon be put to the test. According to their model, at least one of the premerged black holes was spinning rapidly. “That’s something that we have to have in order for our scenario to make any sense,” McKernan says. The team also estimates the mass of the merged black hole to be around 100 times that of the sun. LIGO’s full analysis of the gravitational-wave data will support or refute both assertions. Additionally, as the merged black hole orbits the supermassive one, it will eventually reenter the accretion disk, triggering another flare of light that would be visible from Earth. The team anticipates that this will happen sometime in the summer.
“The main thing I’m excited about is this concrete prediction,” says Katerina Chatziioannou, a member of the LIGO team analyzing the gravitational waves from the event. “I’m waiting to see where that goes.” If the second flare occurs as predicted, it will spark a new era in the study of active galactic nuclei.
Updates in Ultraviolet
Beyond the properties of the black holes themselves, flares of light from merging black holes could reveal properties of the accretion disks in active galactic nuclei. From this first flare, Graham’s team estimated the density of the gas and thickness of the disk—a calculation impossible to make with the gravitational-wave data alone. Future flares could offer other clues about the composition of active galactic nuclei, which “are really critical pieces of our understanding of how galaxies in our universe get built,” Ford says.
But for multimessenger astronomy to flourish, astronomers must update their tools. Although ZTF detected a flare in the visible spectrum, Ford says that flares from other black hole mergers would be easier to spot in ultraviolet wavelengths. Because Earth’s atmosphere absorbs most ultraviolet light before it can reach ground-based observatories, scientists must turn to space telescopes. Graham says nasa’s Neil Gehrels Swift Observatory is the best candidate but an imperfect one: Swift was primarily designed to study gamma-ray bursts. And it has a much smaller field of view than the ZTF telescope.
A handful of other space telescopes that are sensitive to ultraviolet light dot the sky. None is ideal for searching for light flares in the wake of gravitational waves, however. “The ultraviolet has been kind of neglected by the [astronomical] community for a while,” C.U.N.Y.’s Bellovary says. “That’s a problem, honestly.” She and others hope that the new research will spur the development of an ultraviolet telescope with a large field of view. “To have the appropriate satellite would cost a lot of money, but I would certainly advocate that we spend it,” Ford says.
For now scientists await the LIGO results and predicted second flare. Erin Kara, an astrophysicist at the Massachusetts Institute of Technology, finds the black hole merger theory “tempting.” Yet she is not ready to celebrate without more evidence. “Nature comes up with all sorts of crazy things that we can’t explain,” Kara says. “It’s quite likely that we’re just not creative enough to be able to think about all of the possibilities of things that could be causing these flares.”