X-Ray Variability of Active Galactic Nuclei: A Universal Power Spectrum with Luminosity-dependent Amplitude

Abstract

We present power-law model fits to the power spectra of 12 high- quality "long look" observations of AGNs from the EXOSAT data base. The AGNs concerned differ in X-ray luminosity L_x_, over a range 10^4^. The results are consistent with all objects having a power spectrum index equal to the mean value α = 1.55. There is no correlation with L_x_, suggesting that the power spectrum has little curvature over many decades of frequency. The mean slope is inconsistent with both standard shot-noise processes and traditional "l/f noise" but close to the prediction of simple "rotating hot spot" models. The power spectrum amplitude shows large object-to-object scatter but varies systematically as L_x_^-0.5^. This is inconsistent with any fixed-shape shot-noise model. Given the mean power spectrum slope, we then deduce that the time scale which gives fixed power spectral density varies as ~L_x_^0.3^; however, the time scale below which there is fixed integrated power varies almost exactly linearly with luminosity.