Quasi-periodic oscillation in Seyfert galaxies: Significance levels. The case of Markarian 766

We discuss methods to compute significance levels for the existence of quas i-periodic oscillations (QPOs) in active galactic nuclei (AGNs) that take t he red-noise character of the X-ray light curves of these objects into acco unt. Applying epoch folding and periodogram analysis to the XMM-Newton obse rvation of the Seyfert galaxy Mrk 766, a possible QPO at a timescale of 420 0 s has been reported. Our computation of the significance of this QPO, how ever, shows that the 4200 s peak is not significant at the 95% level. We co nclude that the 4200 s feature is an artifact of the red-noise process and not the result of a physical process within the AGNs.