Quasar Variability in the Framework of Poissonian Models

In this paper we review the basic Poissonian formulation of quasarvariability, using it as a mathematical tool to extract relevant parameterssuch as the energy, rate and lifetimes of the flares through the analysis ofobserved light curves. It is shown that in this very general framework the wellestablished anti-correlation between variability amplitude and wavelength canonly be understood as an effect of an underlying spectral component whichremains stable on long time-scales, and is redder than the variable component.The formalism is applied to the B and R light curves of 42 PG quasars collectedby the Wise Observatory group. Variability indices for these data are obtainedwith a Structure Function analysis. The mean number of living flares isconstrained to be in the range between 5 and 100, while their rates are foundto be of order 1--100 per yr. Monochromatic optical flare energies of10^{46-48} erg/A and life-times of 0.5 to 3 yr are derived. Lower limits oftypically 25% are established for the contribution of a non-variable componentin the R band. The substantial diversity in these properties among quasarsinvalidates simple versions of the Poissonian model in which flare energies,lifetimes and the background contribution are treated as universal invariants.The good correlation between the EW(H_beta) and the long term variabilityamplitude is interpreted in a scenario where only the variable componentparticipates in the ionization of the line emitting gas. This idea isconsistent with the observed trends of the variability amplitude with lambda,EW(HeII) and the X-ray to optical spectral index (abridged).Comment: 20 pages (including 10 figures), accepted for publication in Ap