Tick … tick … boom? In the center of a galaxy 1.2 billion light-years from Earth, astronomers say they have seen signs that two giant black holes, with a combined mass of hundreds of millions of Suns, are gearing up for a cataclysmic merger as soon as 100 days from now. The event, if it happens, would be momentous for astronomy, offering a glimpse of a long-predicted, but never witnessed mechanism for black hole growth. It might also unleash an explosion of light across the electromagnetic spectrum, as well as a surge of gravitational waves and ghostly particles called neutrinos that could reveal intimate details of the collision.
As soon as the paper appeared last week on the preprint server arXiv, other astronomers, eager to confirm the tantalizing signals, rushed to secure telescope observing time, says team member Huan Yang of the Perimeter Institute in Waterloo, Canada. “We’ve seen people acting pretty fast,” he says. Emma Kun of Konkoly Observatory in Budapest, Hungary, began to scour archives of radio observations for confirmation of the signal. “If the boom happens, it will confirm many things,” she says.
But the prediction may be a mirage. It’s not clear that the observed galaxy holds a pair of black holes, let alone a pair that’s about to merge, says Scott Ransom of the National Radio Astronomy Observatory, who finds the presented evidence “pretty circumstantial.”
Supermassive black holes are thought to lurk at the heart of most, if not all, galaxies, but theorists don’t know how they grow so big. Some sporadically suck in surrounding material, fiercely heating it and causing the galaxy to shine brightly as a so-called active galactic nucleus (AGN). But the trickle of material may not be enough to account for the black holes’ bulk. They could gain weight more quickly through mergers: After galaxies collide, their central black holes become gravitationally bound and they gradually spiral together.
Such black hole pairs are not easy to detect. X-ray telescopes have discovered a handful of AGNs with two bright, separated central sources, but the putative black holes are hundreds of light-years apart and wouldn’t collide for billions of years. Once they get closer, it’s almost impossible to separate their light with a telescope. But some AGNs regularly dim and brighten, which astronomers have recently argued is a sign they harbor pairs of black holes orbiting each other that regularly churn and heat the surrounding material. Some of these periodic oscillations have faded, however, calling into question the binary interpretation. “AGNs do all sorts of crazy things we don’t understand,” Ransom says.
In data from a survey telescope in California called the Zwicky Transient Facility (ZTF), a team led by Ning Jiang of the University of Science and Technology of China stumbled on a periodic AGN called SDSSJ1430+2303. “My first instinct was it must be related to a pair of supermassive black holes,” Jiang says.
Then, the researchers found something more: a trend they interpret as a binary pair closing in on a merger. The cycles were getting shorter, going from 1 year to 1 month in the space of 3 years. It is “the first official report of decaying periods which reduced over time,” says Youjun Lu, a theoretical astrophysicist at the National Astronomical Observatories of China, who was not part of the team.
The researchers confirmed the monthlong oscillation in x-ray observations from NASA’s orbiting Neil Gehrels Swift Observatory. If this decreasing trend continues, the black holes, which Jiang says come as close to each other as the Sun is to Pluto, will merge in the next 100 to 300 days, they report in the paper, which has not been peer reviewed.
If the merger comes to pass, observers could have a field day. “There should be a huge burst across the electromagnetic spectrum, from gamma rays to radio,” Kun says. Some also expect a flood of neutrinos, which the IceCube detector at the South Pole—1 cubic kilometer of polar ice outfitted with light sensors to detect neutrino impacts—could pick up. Neither, however, is certain. Some predict a whimper rather than a bang. “We really don’t know what to expect,” Ransom says.
The only certain signal is gravitational waves, but the ponderous colliding masses would emit them at too low a frequency to be picked up by detectors such as the Laser Interferometer Gravitational-Wave Observatory, which is tuned to smaller mergers. They should, however, leave an imprint on spacetime itself, a sort of relaxation of distance and time dubbed gravitational wave memory, which could be detected over many years by monitoring the metronomic pulses of spinning stellar remnants known as pulsars. “It’s a very tricky signal to measure,” Ransom says, “but that would be definitive, a total smoking gun” of merging supermassive black holes.
But Ransom is prepared for disappointment. He and others point out the team is basing its prediction on just a handful of observed cycles. Theorist Daniel D’Orazio of the Niels Bohr Institute in Copenhagen, Denmark, says some aspects of the AGN’s light curve also raise doubts. For example, he says, the ZTF archives show SDSSJ1430+2303 lacked a periodic oscillation in the years before Jiang’s team discovered it; its dim, steady emission then looked more like a standard AGN with a single supermassive black hole. “Why has [the oscillation] just turned on now?” D’Orazio asks. “I’m not sure how that steady emission fits with binary emission models.”
Observations in the coming months should show whether the oscillation continues to shorten. The team had to halt its observing in August 2021 when Earth’s orbit put the distant galaxy too close to the Sun for telescopes to observe it safely. Observations restarted in November, but since then technical glitches have idled both ZTF and Swift.
Andrew Fabian of the University of Cambridge is among the astronomers who will be chasing the will o’ the wisp, having applied for time on NASA’s Neutron star Interior Composition Explorer, an x-ray telescope attached to the International Space Station. “If this is true, then it’s important to get as many observations as possible now to see what it’s doing,” he says. Fabian says the chance of such a merger taking place so close to Earth in any given year is one in 10,000. He’s skeptical that one is imminent, but says it’s worth monitoring for a few months to see whether the claim holds up. “Rare events do happen,” he says.