Lognormal X-Ray Flux Variations in an Extreme Narrow-Line Seyfert 1 Galaxy

Abstract

We show that the large variations in the X-ray flux of the extreme narrow-line Seyfert 1 galaxy IRAS 13224Ϫ 3809, measured during a 10 day ASCA observation, have a two-parameter lognormal distribution of multiplicative standard deviation and that the amplitude of variability at any given moment is proportional to the flux j p 2.7 mult 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 13224Ϫ3809 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 j mult give 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 j mult are incompatible with the power-law distribution of flaring amplitudes expected from self-organized critical behavior. We discuss mechanisms that can generate lognormal flux distributions.

Testing the Propagating Fluctuations Model With a Long, Global Accretion Disk Simulation

“…First, GBHB and AGN light curves are log-normally distributed (Gaskell 2004;Uttley et al 2005) which is accounted for through the multiplicative combination of many independent fluctuations. Second, and related to the log-normal distribution, the root-mean square (rms) of the flux variability in the light curves of both GBHBs and AGNs depends linearly on the flux level (Negoro & Mineshige 2002;Gaskell 2004), the so-called rms-flux relationship. This indicates the mass accretion rate variation has a constant fractional amplitude and that the mechanism driving variability is, therefore, independent of the accretion rate (Uttley & McHardy 2001).…”

The minijets-in-a-jet statistical model and the rms-flux correlation

“…These properties were initially studied in the X-ray emission of the galactic black hole binary Cygnus X-1 (Uttley & McHardy 2001;Uttley et al 2005) and are now observed in other accreting objects, such as the blazar BL Lac (Giebels & Degrange 2009), or non-aligned AGNs, such as the Seyfert galaxies NGC 4051 (McHardy et al 2004) and IRAS 13224-3809 (Gaskell 2004).…”

Contribution of Advection Process in SOC Model Causing the Appearance of Lognormal Distribution in Emission

“…There is only additive process in the emission creation, therefore it is expected that the flux distribution will be normal. How can we explain the lognormal flux distribution like those found by Gaskell (2004)?…”

“…However, recent observation of some black hole objects show the existence of multiplicative process in the light fluctuation, which is reflected in its lognormal flux distribution (e.g. Gaskell (2004)). The underlying process in the original SOC model is additive, therefore it is not possible to yield lognormal distribution.…”