Quasar variability in the framework of Poissonian models

Quasar optical monitoring campaigns are reaching a new standard of quality as a result of long-term, accurate CCD observations. In this paper we review the basic Poissonian formulation of quasar variability, using it asa mathematical tool to extract relevant parameters such as the energy, rate, and lifetimes of the hares through the analysis of observed light curves. It is shown that in this very general framework the well-established anticorrelation between variability amplitude and wavelength can only be understood as an effect of an underlying spectral component that remains stable on long tilmescales and is redder than the variable component. The formalism is applied to the B and R light curves of 42 PG quasars collected by the Wise Observatory group. Variability indices for these data are obtained with a structure function analysis. The mean number of living flares is constrained to be in the range between N similar to 5 and 100, while their rates are found to be of order nu similar to 1-100 yr(-1). Monochromatic optical flare energies E-lambda similar to 10(46)-10(48) ergs Angstrom (-1) and lifetimes tau of similar to0.5-3 yr are derived. Lower limits of typically 25% are established for the contribution of a nonvariable component in the R band. The substantial diversity in these properties among quasars invalidates simple versions of the Poissonian model in which flare energies, lifetimes, and the background contribution are treated as universal invariants. Light-curve simulations confirm the applicability of the method. Few significant correlations between variability indices and multiwavelength properties of the quasars exist, confirming the results of Giveon and colleagues in 1999. The good correlation between the EW(HB) and the long-term variability amplitude is interpreted in a scenario in which only the variable component participates in the ionization of the line emitting gas. This idea is consistent with the observed trends of the variability amplitude with lambda, EW(He II), and the X-ray-optical spectral index. The parameter estimates derived under the framework of Poissonian models are applicable to several scenarios for the nature of quasar variability and can help in guiding, testing, and discriminating between detailed physical models.