X-RAY VARIABILITY OF ACTIVE GALACTIC NUCLEI - A UNIVERSAL POWER SPECTRUM WITH LUMINOSITY-DEPENDENT AMPLITUDE

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 alpha = 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 ''1/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 approximately L(x)0.3; however, the time scale below which there is fixed integrated power varies almost exactly linearly with luminosity.