A once-in-a-lifetime event might let off cosmic fireworks, but many people are concerned that the signal will fade.

Tick, tock, tock, tock, tock, tock, t Astronomers believe they’ve detected hints that two gigantic black holes with a combined mass of hundreds of millions of Suns are preparing for a catastrophic merger as soon as 100 days from now in the heart of a galaxy 1.2 billion light-years from Earth. If it occurs, it will be a watershed moment in astronomy, revealing a long-foreseen but never seen process for black hole formation. It might also cause a flash of light over the electromagnetic spectrum, as well as a flurry of gravitational waves and ghostly particles known as neutrinos, which could disclose specific data about the impact.

Other astronomers scrambled to arrange telescope viewing time as soon as the research was published last week on the preprint service arXiv, according to team member Huan Yang of the Perimeter Institute in Waterloo, Canada. “We’ve seen individuals react quickly,” he adds. Emma Kun of Budapest, Hungary’s Konkoly Observatory, started scouring radio archives for confirmation of the transmission. “Many things will be confirmed if the explosion occurs,” she adds.

However, the forecast might be an illusion. According to Scott Ransom of the National Radio Astronomy Observatory, the data offered is “very circumstantial,” and it’s unclear if the detected galaxy has a pair of black holes, much alone a pair that’s going to combine.

Supermassive black holes are assumed to exist at the center of most, if not all, galaxies, but physicists have no idea how they grow to be that massive. Some suck in surrounding material on a periodic basis, intensely heating it and causing the galaxy to flash brightly as a “active galactic nucleus” (AGN). However, the trickle of material may not be sufficient to account for the black holes’ massive size. Mergers might help them acquire weight faster: When galaxies collide, their center black holes become gravitationally bonded and spiral together.

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Such black hole pairings are difficult to see. A number of AGNs with two luminous, separated core sources have been identified by X-ray telescopes, but the putative black holes are hundreds of light-years distant and wouldn’t collide for billions of years. With a telescope, it’s almost hard to distinguish their brightness as they move closer. However, certain AGNs dim and brighten on a regular basis, which scientists have lately said is a hint that they contain pairs of black holes circling one other, churning and heating the surrounding material. However, some of these periodic oscillations have diminished, throwing the binary interpretation into doubt. “AGNs do all kinds of strange things that we don’t understand,” explains Ransom.

A team lead by Ning Jiang of the University of Science and Technology of China discovered a periodic AGN dubbed SDSSJ1430+2303 in data from the Zwicky Transient Facility (ZTF), a survey telescope in California. Jiang explains, “My immediate thought was that it had to do with a pair of supermassive black holes.”

The researchers then discovered something else: a pattern that they interpret as a binary pair on the verge of merging. In the period of three years, the cycles shrank from one year to one month. According to Youjun Lu, a theoretical astrophysicist at China’s National Astronomical Observatories who was not part of the project, it is “the first official record of fading periods that diminished through time.”

The month-long oscillation in x-ray measurements from NASA’s orbiting Neil Gehrels Swift Observatory was verified by the researchers. The black holes, which, according to Jiang, are as near to each other as the Sun is to Pluto, will combine in the next 100 to 300 days if the downward trend continues, they claim in the unpeer reviewed publication.

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Observers may have a field day if the merger goes through. “From gamma rays to radio,” Kun explains, “there should be a massive surge over the electromagnetic spectrum.” Some predict a neutrino flood, which the IceCube detector at the South Pole—a cubic kilometer of polar ice equipped with light sensors to detect neutrino impacts—could detect. Neither is definite, though. Some anticipate that instead of a boom, there will be a whimper. “We have no idea what to anticipate,” admits Ransom.

Gravitational waves are the sole guaranteed signal, but the ponderous clashing masses would release them at a frequency too low for detectors like the Laser Interferometer Gravitational-Wave Observatory, which is tuned to smaller mergers, to detect. They should, however, leave an impression on spacetime itself, a kind of distance and time relaxation known as gravitational wave memory, which might be observed over many years by monitoring the metronomic pulses of spinning star remains known as pulsars. “It’s a difficult signal to quantify,” adds Ransom, “but that would be unequivocal, a complete smoking gun” of merging supermassive black holes.

Ransom, on the other hand, is bracing himself for disappointment. He and others point out that the team’s prognosis is based on just a few cycles that have been witnessed. Some elements of the AGN’s light curve also raise suspicions, according to theorist Daniel D’Orazio of the Niels Bohr Institute in Copenhagen, Denmark. SDSSJ1430+2303, for example, lacked a periodic oscillation in the years before Jiang’s team identified it, according to the ZTF archives; its faint, continuous emission then seemed more like a conventional AGN with a single supermassive black hole. “How come [the oscillation] just started?” D’Orazio inquires. “I’m not sure how that consistent emission fits into binary emission theories,” says the researcher.

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In the next months, observations should reveal if the oscillation continues to shorten. In August 2021, the team had to stop watching since Earth’s orbit brought the distant galaxy too near to the Sun for telescopes to safely view it. Observations resumed in November, but technological issues have kept both ZTF and Swift at a standstill since then.

Andrew Fabian of the University of Cambridge is one of the scientists who has applied for time on NASA’s Neutron Star Interior Composition Explorer, an x-ray telescope linked to the International Space Station, to search for the will o’ the wisp. “If this is true,” he argues, “it’s critical to collect as many observations as possible right now to see what it’s doing.” According to Fabian, the likelihood of such a merger occurring so near to Earth in any given year is one in 10,000. He is doubtful that one is on the way, but believes it is worth keeping an eye on for a few months to see whether the story holds up. “Rare occurrences do occur,” he adds.

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