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 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 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