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 & Courvoisier 1994) has shown that the relative variability of quasars and Seyfert galaxies decreases when the luminosity increases. The spectral information included in the IUE spectra allows us to study this dependence in the rest frame of the objects. The trend is strengthened by the general property that active galactic nuclei varymore at shortwavelengths than at long wavelengths in the ultraviolet domain. The scatter observed in all other studies is still present. An important part of this scatter may however be explained if one tries to estimate the uncertainties on the variability due to the sampling. We discuss the variability 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 most general discrete-eventmodels inwhich the change in average luminosity is due to differences in average event rates exclusively. Several biases are investigated, but we conclude that the −1/2 index is definitely inconsistent with the data. A flat relationship is however possible, if some bias has been underestimated. Wepropose differentwayswherebydiscrete eventsmayproduce a different variability–luminosity relationship: by changing the luminosity or the life time of the events, or by introducing interdependence between the events. The latter possibility cannot produce a satisfactory relationship. Using the former possibilities, we do not find any “natural” explanation for the variability–luminosity relationship in the context of discreteevent models. This is possibly an indication that explanations in which variability is not expressed in terms of discrete events should be favoured.