  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.
