Dependent time lags in the X-ray emission of the Seyfert galaxy NGC 7469

We report the discovery of time lags in the cross spectra of the X-ray light curves of the Seyfert galaxy NGC 7469. This behavior is common in Galactic black hole and neutron star binaries and is in the sense that harder Xrays are delayed with respect to softer X-rays, with a time lag approximately proportional to the Fourier period. At the longest period probed by our observation (similar to6 days), we find a time lag of approximately 3.5 hr between the 2-4 and 4-10 keV X-rays. A similar lag and period dependence are found by comparing the 2-4 and 10-15 keV light curves, albeit with less significance. We find the coherence function of the light curves to be close to 1 in the frequency range between 10(-5.5) and 10(-3.5) Hz. The implications of these results for the X-ray production mechanism in active galactic nuclei are discussed.