REST-FRAME VARIABILITY OF QUASARS AND SEYFERT-GALAXIES IN THE ULTRAVIOLET - CONSTRAINTS ON THE DISCRETE-EVENT MODELS

A study of the ultraviolet continuum variability (Paltani & Courvoisie r 1994) has shown that the relative variability of quasars and Seyfert galaxies decreases when the luminosity increases. The spectral inform ation included in the IUE spectra allows us to study this dependence i n the rest frame of the objects. The trend is strengthened by the gene ral property that active galactic nuclei vary more at short wavelength s than at long wavelengths in the ultraviolet domain. The scatter obse rved in all other studies is still present. An important part of this scatter may however be explained if one tries to estimate the uncertai nties on the variability due to the sampling. We discuss the variabili ty using the concept of discrete events. The trend between variability and luminosity is described by a power-law with an index -0.08, which is incompatible with the power-law of index -1/2 predicted by the mos t general discrete-event models in which the change in average luminos ity is due to differences in average event rates exclusively. Several biases are investigated, but we conclude that the -1/2 index is defini tely inconsistent with the data. A flat relationship is however possib le, if some bias has been underestimated. We propose different ways wh ereby discrete events may produce a different variability-luminosity r elationship: by changing the luminosity or the life time of the events , or by introducing interdependence between the events. The latter pos sibility cannot produce a satisfactory relationship. Using the former possibilities, we do not find any ''natural'' explanation for the vari ability-luminosity relationship in the context of discrete-event model s. This is possibly an indication that explanations in which variabili ty is not expressed in terms of discrete events should be favoured.