## 4. Statistical behaviourThe spectral characteristics of the time series of 0.5-year
coordinates in the source-fixed frames are investigated using the
Allan variance method (Allan 1966, see a review of these methods in
Rutman 1978). The Allan variance analysis allows one to characterize
the power spectrum of the variabilities in time series, for sampling
times ranging from the initial interval of the series to 1/4 to 1/3 of
the data span, in our case 6 months to 3-4 years. This method
identifies white noise (spectral density S independent of frequency
f), flicker noise (S proportional to f
Most sources show similar stability sigantures in both the x- and y-directions up to four years sampling time. Four sources, 1611+343, 1633+382, 1739+522, and 1803+784, show a remarkable discrepancy between flicker noise behaviour in the x-direction and marked white noise behaviour, at a much lower level, in the y-direction. Considering the Standard Model (deceleration parameter =0.5, cosmological constant =0) and a value =60km/s/Mpc for the Hubble constant, one can compute distances of objects as a function of the redshift (Mattig 1958), and hence transform angular dimensions into linear ones (Theureau 2000). Table 4 summarizes the characterization of the sources stability in angular and linear units.
© European Southern Observatory (ESO) 2000 Online publication: July 13, 2000 |