The ultraviolet and X-ray spectrum of NGC 5548 : evidence for thermal Comptonization and geometrical variations.

Abstract

Ultraviolet, soft and hard X-ray observations of NGC 5548 performed by IUE and EXOSAT give us a clear picture for the geometry of the emitting source. This picture is provided by the correlations observed between the large variations of the ultraviolet continuum, of the soft and hard X-ray fluxes, of the hard X-ray powerlaw index and of the equivalent width of the 6.5 keV Iron line. The modifications of the powerlaw index and of the X-ray fluxes show that the hard X-ray photons are emitted by optically thin thermal comptonization of soft X-ray photons. The distance comprised between the hard X-ray emitting cloud and the accretion disk enlarges with the increasing central activity. The whole set of X-ray long term variations and probably a part of the broad emission line variations are related to this distance.