NGC 5548 : a perfect laboratory for testing AGN models ?

Motivated by the recent Ginga monitoring campaign of NGC 5548, we continue the study undertaken by Rokaki & Magnan (1992) of the correlated variations of the optical-UV continuum and of the line fluxes in NGC 5548. This work is done in the framework of the "accretion disc model" of AGN. A fundamental assumption of the model is that an important fraction of the bolometric luminosity is emitted in the X-ray range (and possibly also in the IR-millimeter range) by a central source, and provides a major part of the heating and emission of the disc. One fraction hits directly the inner regions of the disc and results in optical-UV continuum emission, while another fraction is backscattered by a hot medium towards the outer regions, and is reprocessed mainly into line emission. The optical-UV continuum and the line respond to X-ray flux variations with different time delays which set strong constraints on the size of the continuum and line emission regions. The present study differs from the previous one in several respects. First the scattering medium which was schematized by a point-like source is now correctly treated, and different physical situations are envisioned. Second, 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 object. Two important conclusions emerge. The structure of the hot diffusing medium is found to be that of a "mass loaded flow" located relatively close to the black hole. This study also confirms the finding that, either accretion through the disc represents only a marginal portion of the total accretion rate, or the disc accretion rate supply is mainly converted into X-ray and not into UV radiation. Finally several issues are discussed, such as the "Baldwin effect" due to flux variations, or predictions on the light pattern at different wavelengths.