NGC-5548 - A PERFECT LABORATORY FOR TESTING AGN MODELS

Motivated by the recent Ginga monitoring campaign of NGC 5548, we cont inue the study undertaken by Rokaki & Magnan (1992) of the correlated variations of the optical-UV continuum and of the line fluxes in NGC 5 548. This work is done in the framework of the ''accretion disc model' ' of AGN. A fundamental assumption of the model is that an important f raction of the bolometric luminosity is emitted in the X-ray range (an d possibly also in the IR-millimeter range) by a central source, and p rovides a major part of the heating and emission of the disc. One frac tion hits directly the inner regions of the disc and results in optica l-UV continuum emission, while another fraction is backscattered by a hot medium towards the outer regions, and is reprocessed mainly into l ine emission. The optical-UV continuum and the line respond to X-ray f lux variations with different time delays which set strong constraints on the size of the continuum and line emission regions. The present s tudy differs from the previous one in several respects. First the scat tering medium which was schematized by a point-like source is now corr ectly treated, and different physical situations are envisioned. Secon d, the different constraints imposed on the physical parameters of the model are discussed more extensively. Third, we test the assumptions and the inferred physical parameters of the model using the results of the recent X-ray, UV and optical observations carried out on this obj ect. Two important conclusions emerge. The structure of the hot diffus ing medium is found to be that of a ''mass loaded flow'' located relat ively close to the black hole. This study also confirms the finding th at, either accretion through the disc represents only a marginal porti on of the total accretion rate. or the disc accretion rate supply is m ainly converted into X-ray and not into UV radiation. Finally several issues are discussed, such as the ''Baldwin effect'' due to flux varia tions, or predictions on the light pattern at different wavelengths.