Radio variability in complete samples of extragalactic radio sources at 1.4 GHz.

Complete samples of extragalactic radio sources (GB, GB 2, GB 3; for references cf. Rys and Machalski, 1987) obtained in 1970-1975, and the sky survey of Condon and Broderick made in 1983 were used to select sources variable at 1.4 GHz, and to investigate characteristics of variability in the whole population of sources at this frequency. The radio structures, radio spectral types, and optical identifications of the selected variables are discussed. Only compact flat-spectrum sources vary at 1.4 GHz, and all but four are identified with QSOs, BL Lacs, or other (unconfirmed spectroscopically) stellar objects. No correlation of degree of variability at 1.4 GHz with galactic latitude or variability at 408 MHz (cf. Gregorini et al., 1986) has been found, suggesting that most of the 1.4 GHz variability is intrinsic and not caused by refractive scintillations. Numerical models of the variability have been computed. From these models, assuming that individual bursts of radiation with similar intensity profiles are superimposed on a quiescent level of flux, we found that (1) the intrinsic amplitudes A of the fractional flux-density variations have a power-law distribution φ(A) is proportional to A ^-2.5 =/- 0.25^, (2) the fraction of intrinsically variable sources in the whole population at 1.4 GHz is not smaller than 0.07 and not larger than 0.20, so at least 1/3 of all flat-spectrum sources must be variable at 1.4 GHz, and (3) a mean time scale of about 7-16 yr is suggested by the models for individual bursts of enhanced radiation.