At the heart of galaxy 1ES 1927+654, a black hole behaves like a lighthouse with a faulty mechanism. Every six and a half minutes, it releases a burst of X-ray light, a rhythm astronomers have been tracking since 2022. At first, the beacon's tempo wandered — speeding up and slowing down, with intervals ranging from one beat every eighteen minutes to one every seven. Then, as if a technician had tightened the spring, the pulse settled into a steady rhythm.

The XMM-Newton telescope monitored the galaxy for a year and a half, accumulating over nine hundred of these 'beats'—a record for such objects. The pulse isn't just regular; it lacks the fainter secondary echo often seen in pulsating sources. Instead, there's a curious delay: soft X-rays arrive slightly after hard ones, and this lag remains constant even when the rhythm itself tried to falter. It's as if the lighthouse has two mirrors (one producing the flash, the other its reflection) and the second always lags by the same amount.

The NuSTAR telescope has now caught the same pulse in harder X-rays for the first time. Two different instruments seeing the same heartbeat means it's real, not a data artifact. What exactly drives the black hole's pulsation remains unclear. Perhaps a dense clump of gas orbits nearby, periodically blocking the light like a hand in front of a flashlight. Or the corona, the hot region above the black hole, is 'breathing,' expanding and contracting. No model yet explains all three oddities at once: the steady rhythm, the delayed echo, and the sharp brightness swings.