2004ApJ...612L..21G

We show that the large variations in the X-ray flux of the extreme narrow-line Seyfert 1 galaxy IRAS 13224-3809, measured during a 10 day ASCA observation, have a two-parameter lognormal distribution of multiplicative standard deviation σ_mult=2.7 and that the amplitude of variability at any given moment is proportional to the flux level. We find similar behavior in earlier ROSAT soft X-ray monitoring. There is no evidence of a nonvariable component. The flux-dependent behavior of the variability rules out linear shot-noise models. Although at first glance the variations of the ASCA light curve for IRAS 13224-3809 appear to exhibit nonstationary behavior with quiescent low states and more active flaring high states, our results show that the multiplicative variance is constant. Monte Carlo simulations of constant σ_multgive excellent matches to the observed X-ray light curve without the need to invoke special low and high states. This supports a picture in which the long-term variability is fundamental both in active galactic nuclei and in X-ray binaries. The lognormal flux distribution and the constancy of σ_multare incompatible with the power-law distribution of flaring amplitudes expected from self-organized critical behavior. We discuss mechanisms that can generate lognormal flux distributions.