A ~1-hour X-ray periodicity in an active galaxy RE J1034+396

Active galactic nuclei and quasars are thought to be scaled up versions of Galactic black hole binaries, powered by accretion onto supermassive black holes with masses of 10^6-10^9 M_Sun, as opposed to the ~10 M_Sun in binaries. One example of the similarities between these two types of systems is the characteristic rapid X-ray variability seen from the accretion flow. The power spectrum of this variability in black hole binaries consists of a broad, band-limited noise with multiple quasi-periodic oscillations superimposed, where power is concentrated over a narrow range of frequencies. Although the broad noise component has been observed in many active galactic nuclei, there are no significant detections of quasi-periodic oscillations. Here we report the discovery of a ~1h X-ray periodicity in a bright active galaxy RE J1034+396. The signal is highly statistically significant (at the 5.6 sigma level) and very coherent, with quality factor Q > 16. This reinforces the link between stellar and supermassive black holes, emphasizing the universal properties of accretion onto objects with very different masses. The X-ray modulation arises from the direct vicinity of the black hole, so this provides a new tool for studying active galactic nuclei.