SIMBAD references

We have analysed all spectra of all active galactic nuclei (AGN) observed 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 objects the flux variations are more important at 1200 A (mean of 39%) than at 3200A (mean of 21%). Rather small differences are 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 settled between the variability properties of the radio-quiet and radio-loud quasars. We found however that the BL Lac objects have a larger fraction of high-variability objects than the other objects. All classes of objects present an increase of variability towards short wavelengths. We found no strong connections between the variability properties and the other observational parameters of the objects, like flux, redshift, luminosity or ultraviolet spectral index. Nevertheless we are able to put some constraints on the existing models. The models based on independent events with the same luminosity are ruled out. The variability is also anticorrelated to the luminosity in the Seyfert-like objects.