In the distant galaxy OJ 287, there are two black holes. One is enormous, as massive as 18 billion Suns. The second is smaller, but still a hundred million times heavier than our star. The smaller hole orbits the larger one like a pendulum in an old clock. Every 12 years it plows through the gas disk surrounding the big hole, and the clock strikes a beat: a bright flash of light.
In October 2022, the pendulum swung again. The smaller hole tore through the disk and flung out a clump of gas. That clump headed toward the jet, a narrow beam of plasma shooting from near the big hole. A few months later, the clump reached the jet and made it flare up. Telescopes saw the light in three bands at once: visible, ultraviolet, and X-ray.
Previously, scientists debated what kind of flare occurred in October 2022. Now the debate is settled. The flare turned out to be tidal, just as the binary black hole model predicted. This matters because tidal flares allow a check on the big hole's mass. Earlier estimates came from how the small hole's orbit precesses, yielding 18.35 billion solar masses. Now that figure has been confirmed by another method: the width of the hydrogen line in the spectrum. The two estimates agree.
The clock keeps ticking. The next passage of the small hole through the disk will be in about 60 years. Astronomers hope to find old photographic plates with past flares to fine-tune this cosmic clock even further.
OJ 287 has been observed since 1888, and its 137-year-long light curve contains flares predicted by the model decades in advance. The seven brightest flares in history include four impact flares and three tidal flares, with some historical flares visible only as tidal due to gaps in observations.
The idea of a binary supermassive black hole periodically flaring as it crosses an accretion disk echoes the image of a 'black hole lighthouse' from science fiction, such as in Greg Egan's novel 'The Clockwork Rocket', where orbiting bodies around black holes produce predictable emissions.