Physical interpretation of the UV/X-ray variability behavior of NGC 7469

Abstract

We present a re-analysis of the simultaneous 30 day IUE/XTE observation of NGC 7469 done in 1996. Our main progress, in comparison to previous spectral analysis, is to adopt and fit directly to the data a detailed Comptonization model. This firstly allows to fit simultaneously the data from the UV to the hard X-ray band in a self-consistent way and secondly it gives direct constraints on the physical parameters of the disc-corona system. Our results are completely consistent with a slab geometry where all the observed UV emission is supposed to cross the corona but more photon-starved geometries also give acceptable fits. Whatever the geometry is, the UV seed photon emission appears to be dominated by the reprocessing of the X-rays. We also found very interesting correlations between the different model parameters, the most important one being the anticorrelation between the corona temperature and the UV flux. Such an anticorrelation is clearly inconsistent with a fixed disc-corona configuration and suggests a variation of the geometry of the system. We also find a correlation between the corona optical depth and the X-ray flux which may reflect processes linked to the corona formation. During the observations, NGC 7469 appears to accrete near its Eddington limit. This source could then be an example of magnetically dominated disc-corona system as recently proposed by Merloni (2003).