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.
as written by the authors arXiv:2604.16688
1ES 1927+654 is an extreme active galactic nucleus (AGN) that has defied our canonical expectations for how AGN appear across the electromagnetic spectrum and how they vary on short timescales. In 2022, this source began showing a X-ray quasi-periodic oscillation (QPO) at mHz frequencies, along with a newly launched radio jet. Unlike the handful of other known AGN QPOs, the QPO in 1ES 1927+654 showed a significant frequency evolution, spanning from 0.9-2.4 mHz from 2022-2024. In this work, we present the last 1.5 years of monitoring with XMM-Newton (250 ks) up to January 2026, which reveals that the QPO persists but has plateaued at a constant frequency of approximately 2.5 mHz. We perform detailed spectral-timing analyses on this exquisite dataset, consisting of over 900 QPO cycles, more than any AGN QPO to date. Our main findings are: (1) the stacked XMM-Newton power spectra shows no significant second harmonic, (2) a soft (reverberation-like) lag is observed at all frequencies and remains remarkably stable even as the QPO frequency evolved from 2022-2024, and (3) extreme X-ray jumps on the QPO period (up to ~80% baseline flux) persist to present day with a remarkably stable dip-rise-fall pattern. Finally, we also detect the first AGN QPO in NuSTAR observations, which is present from 2023 to 2026 at frequencies consistent with the XMM-Newton detections. While we explore models for eclipses and coupled disk-corona behavior to simultaneously explain the lags, dips, and QPO, these new observations strain such models.
During the observations, this pulse has 'ticked' more than nine hundred times. If each beat were a step, the black hole would have walked from Earth to the Moon.
In Stanislaw Lem's novel 'Solaris,' the planet's ocean pulsates with a mysterious regularity that scientists cannot explain — much like this black hole's heartbeat.