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Astron. Astrophys. 359, 1201-1204 (2000)

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2. The astrometry data set

The astrometry data set used in this study is a time series of session-per-session source positions computed in a homogeneous reference frame from observations going back to August 1979.

For historical reasons, the numbers of observations per source are extremely uneven. Some sources that were used for providing reference directions in the early years of VLBI appeared to be too variable or too extended after some years and were discarded to the benefit of other, fainter sources that became usable for astrometric purposes thanks to the progress in technology. Some source considered as best fitting the astro-geodetic needs are repeatedly observed, while others, that are less useful for this purpose, are re-observed less frequently, mainly for astronomical studies.

According to Ma et al. (1998) there are two major causes for the current limitation in accuracy of source positions.

  • The first cause is connected to the atmospheric delay correction combined with the geometry of the observing networks. The uncertainty in the modelling of the propagation delay due to the wet component of the troposphere, combined with deficiencies in the network geometry (majority of stations in the northern mid- latitudes, short N-S components of the baselines yielding to the observation of low declination objects at low elevation, where the mis-modelling of the delay has the largest effect), may give rise to systematic errors in declination at the level of 0.5 mas (milliarcsecond) in the zone around the equator.

  • The second cause is related to the fact that no radiosource is really point-like when observed in centimetric wavelength with baseline lengths around 6000 km. If the source structure is extended or not circular, its apparent direction may change as a function of the length and orientation of the baselines. This effect should be minimized with the practice of running 24-hour sessions, during which the Earth's sidereal rotation leads to the diversification of the projections of the baselines on the source structure. Moreover, despite the selection of quiet objects for astro-geodetic work, any of them may exhibit changes in their emission structure that will make their apparent direction change with time. In principle it is possible to accurately correct this effect, provided that repeated maps of the sources are available (Charlot & Sovers, 1997). In the framework of the ICRF maintenance, a systematic mapping program is under way (Fey & Charlot 1997, 2000; http://www.usno.navy.mil/RRFID ). However, the overall correction procedure is not yet implemented in the existing global analysis softwares. This mismodelling may propagate errors into the source positions at the level of 0.2 mas.

The latter effect is precisely the one we investigate in this study, based on the computed coordinates of the radiosources in a homogeneous reference frame, with one determination for each of the sessions in which the source was observed.

In order to be able to use standard time series statistics, the original series with irregular spacing is first transformed into equally spaced time series by performing weighted averages. Within the set of original sources, only those that could provide a continuous (or quasi-continuous) series of right ascensions and declinations at 6-month intervals over 1988-1999 are considered. In addition, in order to avoid the zone where declination instabilities due to inaccurate modelling of the tropospheric wet delay could mix with actual source instabilities, we further kept only sources north of +20 degrees in declination. A total of 16 sources passed successfully this double selection. They are listed in Table 2, together with information on the type of objects, redshift and flux density taken from Véron & Véron (1998), and the standard deviations of half-yearly positions with respect to the mean over the total time interval, in right ascension and declination.


[TABLE]

Table 2. 16 radiosources continuously observed over 1988-1999.
Notes:
1) Spectral classification according to Véron & Véron (1998) [HP: High optical polarization ([FORMULA] 3%); S1.0, S2.0: Intermediate Seyfert galaxies; BL Confirmed BL Lac object.]


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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000
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