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

¹ Laboratoire d'Astrophysique de Grenoble, BP 43, 38041 Grenoble Cedex 9, France
² Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: maraschi@brera.mi.astro.it
³ Dipartimento di Scienze, Universitá dell'Insubria, via Lucini 11, 22100 Como, Italy e-mail: Francesco.Haardt@mib.infn.it
⁴ Astrophysics group, Imperial College London, Blackett Laboratory, Prince Consort Road, London SW7 2AZ, UK e-mail: k.nandra@imperial.ac.uk

Corresponding author: P. O. Petrucci, petrucci@obs.ujf-grenoble.fr

Received: 21 February 2003
Accepted: 15 September 2003

Abstract

We present a re-analysis of the simultaneous ~30 day IUE/XTE observation of NGC 7469 done in 1996. Our main progress in this paper, in comparison to previous spectral analyses (Nandra et al. [CITE]), is to adopt and fit directly to the data a detailed model of the Comptonized spectrum. This firstly allows one 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, like the temperature and optical depth of the corona. 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, 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 kT_e and the UV flux F_UV. 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 ([CITE]). Finally, these data strongly support the presence of a pair free corona.