Frequency-dependent Time Lags in the X-Ray Emission of the Seyfert Galaxy NGC 7469

Abstract

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 X-rays 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 (~6 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.