High-frequency variability of extragalactic radio sources
III. The time scale of variability
Pawel Magdziarz
Astronomical Observatory of the Jagiellonian University, PL-30244
Cracow, Poland
The mean time scales of radio sources variability at cm-wavelengths
predicted for the total population of radio sources from the statistical
model (Machalski & Magdziarz 1993; Magdziarz & Machalski
1993) were examined using the structure function analysis. The
statistical model assumes that the observed variability is due
to a superposition ofindividual radiation bursts having similar
intensit profiles. It is shown that the mean time scale predicted
from the model does not contradict observations, and fully agrees
with the scale derived for the complete flux density limited sample
of Seielstad et al. (1983). A mean durationtime of a burst radiation
(which corresponds to an exponential decaying time) of $<\tau
>=6\pm 2$yrs was found at the frequency of 10.8 GHz. It is
suggested that the mean time scale, characterizing the total population
of radio sources, should be greater than the value of about 2yrs
obtained from observations of well-studied variables (e.g. Altschuler
1989; Hughes et al. 1992). This discrepancy is likely due to assuming
implicitly that already known, well-studied variables are representative
for the total population of sources. The limited time base of
observations, and underestimated contribution of long-term variables
may be responsible for strong selection effects in the observational
data. The general statistical character of variability, as suggested
in the analysis of UMRAO database by Hughes et al. (1992) was
found to beconsistent with the predictions of the model.
Key words - galaxies: active--radio sources:general--variability