The X-ray continuum time-lags and intrinsic coherence in AGN

We present the results from a systematic analysis of the X-ray continuum ('hard') time-lags and intrinsic coherence between the 2-4 keV and various energy bands in the 0.3-10 keV range, for 10 X-ray bright and highly variable active galactic nuclei (AGN). We used all available archival XMM-Newton data, and estimated the time-lags following Epitropakis & Papadakis. By performing extensive numerical simulations, we arrived at useful guidelines for computing intrinsic-coherence estimates that are minimally biased, have known errors and are (approximately) Gaussian distributed. Owing to the way we estimated the time-lags and intrinsic coherence, we were able to do a proper model fitting to the data. Regarding the continuum time-lags, we are able to demonstrate that they have a power-law dependence on frequency, with a slope of -1, and that their amplitude scales with the logarithm of the light-curve mean-energy ratio. We also find that their amplitude increases with the square root of the X-ray Eddington ratio. Regarding the intrinsic coherence, we found that it is approximately constant at low frequencies. It then decreases exponentially at frequencies higher than a characteristic 'break frequency'. Both the low-frequency constant intrinsic-coherence value and the break frequency have a logarithmic dependence on the light-curve mean-energy ratio. Neither the low-frequency constant intrinsic-coherence value nor the break frequency exhibits a universal scaling with either the central black hole mass or the X-ray Eddington ratio. Our results could constrain various theoretical models of AGN X-ray variability.