2001ApJ...554..710N

We present a study of the time variability of the Seyfert 1 galaxy NGC 7469, based on a ∼30 day RXTE/PCA observation. Variability is seen across the X-ray band, and spectral changes are observed. The softness ratio is correlated with the ultraviolet flux but appears to show more rapid variability. The rms variability parameter calculated on ∼1 day timescales is significantly variable, and sharp increases are seen close to the periods during which the spectrum is hardest. Cross correlation of 2-4, 4-10, and 10-15 keV light curves show a peak at zero delay, but the CCFs are skewed toward positive lags (i.e., the soft leading the hard), with an apparent time lag of about 0.5 days. The power spectral density (PSD) function in the 2-10 keV band shows no clear features–for example periodicities or breaks–but a power law is a rather poor fit, particularly to the high-frequency spectrum. The normalized PSD in the soft X-ray band shows a larger amplitude of variability on long timescales, but the hard X-ray PSD is flatter and shows more power on timescales less than about 1 day. Our data broadly support the idea that the X-rays are produced by Compton upscattering of lower energy seed photons, as had been previously concluded. They are difficult to reconcile with models in which the sole source of variability is the seed photons, and more likely suggest a variability process intrinsic to the X-ray corona. Our interpretation for these results is that the low-frequency variability in NGC 7469 does arise via variations in the Compton seed photons but that the high-frequency variability arises from the coronal heating mechanism. Both induce spectral variations. A patchy corona is implied, and one interpretation consistent with the data is that hotter blobs exist closer to the central black hole, responsible for the most rapid variations in the hard X-rays.