HIGH-FREQUENCY VARIABILITY OF EXTRAGALACTIC RADIO-SOURCES .3. THE TIME-SCALE OF VARIABILITY

The mean time scales of radio sources variability at cm wavelengths pr edicted for the total population of radio sources from the statistical model (Machalski and Magdziarz 1993; Magdziarz and Machalski 1993) we re examined using the structure function analysis. The statistical mod el assumes that the observed variability is due to a superposition of individual radiation bursts having similar intensity profiles. It is s hown that the mean time scale predicted from the model does not contra dict observations, and fully agrees with the scale derived for the com plete flux density limited sample of Seielstad et al. (1983). A mean d uration time of a burst radiation (which corresponds to an exponential decaying time) of [tau] = 6+/-2 yr was found at the frequency of 10.8 GHz. It is suggested that the mean time scale, characterizing the tot al population of radio sources, should be greater than the value of ab out 2 yr obtained from observations of well-studied variables (e.g. Al tschuler 1989; Hughes et al. 1992). This discrepancy is likely due to assuming implicitly that already known, well-studied variables are rep resentative for the total population of sources. The limited time base of observations, and underestimated contribution of long-term variabl es may be responsible for strong selection effects in the observationa l data. The general statistical character of variability, as suggested in the analysis of UMRAO database by Hughes ct al. (1992) was found t o be consistent with the predictions of the model.