SYSTEMATIC STUDY OF THE ULTRAVIOLET VARIABILITY OF ACTIVE GALACTIC NUCLEI

We have analysed all spectra of all active galactic nuclei (AGN) obser ved with the IUE satellite during the period 1978-1991. 90 % of the 93 objects for which an estimate of the variability can be obtained have varied by more than 15 % in the ultraviolet domain. In almost all obj ects the flux variations are more important at 1200 Angstrom (mean of 39 %) than at 3200 Angstrom (mean of 21 %). Rather small differences a re found between the different classes of objects. In particular, the dispersions of the variability parameters in each class of objects are larger than the class-to-class differences. No difference can be sett led between the variability properties of the radio-quiet and radio-lo ud quasars. We found however that the BL Lac objects have a larger fra ction of high-variability objects than the other objects. All classes of objects present an increase of variability towards short wavelength s. We found no strong connections between the variability properties a nd the other observational parameters of the objects, like flux, redsh ift, luminosity or ultraviolet spectral index. Nevertheless we are abl e to put some constraints on the existing models. The models based on independent events with the same luminosity are ruled out. The variabi lity is also anticorrelated to the luminosity in the Seyfert-like obje cts.