Short time-scale AGN X-ray variability with EXOSAT: black hole mass and normalized variability amplitude

The old EXOSAT medium energy measurements of high frequency (HF) AGN power spectral normalisation are re-examined in the light of accurate black hole mass determinations which were not available when these data were first published (Green et al 1993). It is found that the normalised variability amplitude (NVA), measured directly from the power spectrum, is proportional to M^{beta} where beta ~ -0.54 +/- 0.08. As NVA is the square root of the power, these observations show that the normalisation of the HF power spectrum for this sample of AGN varies very close to inversely with black hole mass. Almost the same value of $\beta$ is obtained whether the quasar 3C273 is included in the sample or not, suggesting that the same process that drives X-ray variability in Seyfert galaxies applies also to 3C273. These observations support the work of Gierlinski et al (2008) who show that an almost exactly linear anticorrelation is required if the normalisations of the HF power spectra of AGN and X-ray binary systems are to scale similarly. These observations are also consistent with a number of studies showing that the short timescale variance of AGN X-ray lightcurves varies approximately inversely with mass.