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Active galactic nuclei and quasars are thought to be scaled up versions ofGalactic black hole binaries, powered by accretion onto supermassive blackholes 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 thecharacteristic rapid X-ray variability seen from the accretion flow. The powerspectrum 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 thebroad noise component has been observed in many active galactic nuclei, thereare no significant detections of quasi-periodic oscillations. Here we reportthe discovery of a ~1h X-ray periodicity in a bright active galaxy REJ1034+396. The signal is highly statistically significant (at the 5.6 sigmalevel) and very coherent, with quality factor Q > 16. This reinforces the linkbetween stellar and supermassive black holes, emphasizing the universalproperties of accretion onto objects with very different masses. The X-raymodulation arises from the direct vicinity of the black hole, so this providesa new tool for studying active galactic nuclei.

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