Fractal X-ray time variability and spectral invariance of the Seyfert galaxy NGC5506

Abstract

Rapid X-ray variability has long been sought after in active galactic nuclei (AGN) as an indicator of such parameters as the mass-to-energy conversion efficiency and black hole mass. Although some interesting variability information has been gained from early observations (for example see ref. 1 for a review) the faintness of AGN, combined with the short times devoted to their observation, has resulted in a very sparsely sampled and consequently partially misunderstood picture of AGN variability. The launch of EXOSAT² in May 1983 provided us with the first opportunity to study seriously AGN X-ray variability and significant variations have been detected in a number of short observations^3,4. Here we report the results of a long (3-day) observation of NGC5506 in which continuous rapid variation was detected. Power spectral analysis and fractal analysis both demonstrate that the variability is self-similar, that is, scale invariant, over nearly three decades of frequency, a range similar to that covered by pre-EXOSAT observations of bright galactic sources. If this behaviour is typical of all AGN then the concept of a 'two-folding' timescale is meaningless unless the fractal dimension of the general variability, as well as the specific time interval over which a specific change in flux occurs, are all noted. There is no evidence for any variation in the X-ray spectrum as a function of intensity and we show how this places strong constraints on the emission mechanism, favouring non-thermal models involving shocks.