High-frequency variability of extragalactic radio sources. II: A statistical multi-frequency model of variability

Astronomy and Astrophysics – Astrophysics

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The numerical model of extragalactic variability, proposed by Rys & Machalski (1990), is extended for multi-epoch and multi-frequency sampling of an imaginary population of variable sources. Variability observations gathered in Paper I of this series (cf. Introduction) are used to constrain free parameters of the model. The fits to the observations are satisfactory if the distributions of burst amplitude, duration, and recurrence time between consecutive bursts of radiation are frequency-dependent. The model shows how the characteristics of variability depends on the time-filter applied in observations. In particular we found that (1) the intrinsic amplitude A of the flux- density fluctuations varies with frequency as ν^0.41+/-0.14^, (2) the mean timescale of variability <ω>, characterizing the total population of variables, varies as ν^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 loss of identity and dissolution of the burst in slowly decaying previous bursts, (3) the "intrinsic" fraction of variables (i.e. a fraction independent on the time filter applied) should increase (e.g. for sources with apparent fluctuation > 0.3, about five times) with increasing frequency from 1.4 to 10.8 GHz, (4) (i) the mean timescale observed in a sample of variables, ω^bar^, is shorter than that in the total population, and (ii) the observed fraction of variables is lower than the intrinsic one, if the time base of observations is shorter than (0.4-1.0) <ω> (depending on a number of sampling epochs).

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