The variable component of the blue - bump in quasars and SeyfertgalaxiesST

The ultraviolet spectral slope of individual Seyfert 1 galaxies and quasars, measured using IUE observations,varies considerablywith time. Neither accretiondisks nor opticallythin emission models can explain these variations. A superimposition of two components can easily solve the problem. We propose here a method based on the variability properties of these objects to disentangle various components of constant spectral shape. This method is applied to the IUE spectra of a set of quasars and Seyfert galaxies. A variable component and a stable component are thereby obtained for each object. The variable component can be explained by reprocessing on an accretion disk or by free-free emission, but with some diiculties. The variable component of 3C 273 also has been estimated in the optical domain. It is not compatible with optically thin emission, but marginally compatible with reprocessing on an accretion disk.