Far away in the constellation Cancer, 3.5 billion light-years from Earth, a pendulum clock is ticking. The pendulum is a secondary black hole orbiting a much more massive primary. Every twelve years, the pendulum swings through the gas disk surrounding the central black hole, and the clock strikes the hour: a bright flare of light that telescopes can see.

In October 2022, the clock chimed again. But this time, the flare was special. As the secondary black hole plowed through the disk, it didn't just strike the disk like a hammer on a bell; it also ejected a clump of matter that headed toward the jet, a narrow beam of plasma shooting from the black hole's vicinity. Months later, that clump reached the jet and made it flare in optical, ultraviolet, and X-ray light. Such an event is called a tidal flare.

Previously, there was a debate about this flare. Some thought the secondary black hole simply struck the disk like a hammer on a bell, causing an impact flare. Others insisted on the tidal scenario. The new observations settled the debate: the flare was tidal, just as the binary black hole model had predicted.

This matters because tidal flares allow an independent check on the primary black hole's mass. Earlier estimates came from orbital precession, the slow rotation of the orbital plane of the secondary black hole. That gave 18.35 billion solar masses. Now this number has been confirmed by a different method: the width of the hydrogen Hβ spectral line, which is related to the motion of gas around the black hole. The two estimates agree within uncertainties.

The pendulum clock keeps ticking. The next passage of the secondary black hole through the disk is expected in about sixty years. By then, astronomers hope to find old photographic plates of past flares to refine the orbit and further test how accurately this cosmic clock keeps time.