Components of the ultraviolet continuum in active galactic nuclei

The ultraviolet spectral slope of individual active galactic nuclei (AGN), measured using IUE observations, varies considerably with time. Neither accretion disks nor optically thin emission models can explain these variations. We propose here a method based on the variability properties of these objects to disentangle various components of constant spectral shape. A variable component and a stable component are thereby obtained. The varying component is power-law shaped with evidence of reddening. The stable component can be reproduced with the sum of a steep Balmer continuum and of a Fe II pseudocontinuum. Although accretion disk, reprocessing and thermal bremsstrahlung models are compatible with the spectral shape of the varying component in most objects, these simple models cannot explain all the observational constraints.