HIGH-FREQUENCY VARIABILITY OF EXTRAGALACTIC RADIO-SOURCES .2. A STATISTICAL MULTIFREQUENCY MODEL OF VARIABILITY

The numerical model of extragalactic variability, proposed by Rys & Ma chalski (1990), is extended for multi-epoch and multi-frequency sampli ng of an imaginary population of variable sources. Variability observa tions gathered in Paper I of this series (cf. Introduction) are used t o constrain free parameters of the model. The fits to the observations are satisfactory if the distributions of burst amplitude, duration, a nd recurrence time between consecutive bursts of radiation are frequen cy-dependent. The model shows how the characteristics of variability d epends on the time-filter applied in observations. In particular we fo und that (1) the intrinsic amplitude A of the flux-density fluctuation s varies with frequency as nu0.14+/-0.14, (2) the mean timescale of va riability [omega], characterizing the total population of variables, v aries as mu-0.9+/-0.1, and should increase from about 10-15 yr at 10.8 GHz to about 80-120 yr at 1.4 GHz. This behavior is explained by a lo ss of identity and dissolution of the burst in slowly decaying previou s bursts, (3) the ''intrinsic'' fraction of variables (i.e. a fraction independent on the time filter applied) should increase (e.g. for sou rces with apparent fluctuation >0.3, about five times) with increasing frequency from 1.4 to 10.8 GHz, (4) (i) the mean timescale observed i n a sample of variables, omegaBAR, is shorter than that in the total p opulation, and (ii) the observed fraction of variables is lower than t he intrinsic one, if the time base of observations is shorter than (0. 4-1.0) [omega] (depending on a number of sampling epochs).