On characterizing the variability properties of X-ray light curves from active galaxies

We review some practical aspects of measuring the amplitude of variability in ‘red noise’ light curves typical of those from active galactic nuclei (AGN). The quantities commonly used to estimate the variability amplitude in AGN light curves, such as the fractional rms variability amplitude, Fvar, and excess variance, σ2XS, are examined. Their statistical properties, relationship to the power spectrum and uses for investigating the nature of the variability processes are discussed. We demonstrate that σ2XS (or similarly Fvar) shows large changes from one part of the light curve to the next, even when the variability is produced by a stationary process. This limits the usefulness of these estimators for…