1 Introduction

Very-long-baseline interferometry (VLBI) allows routine imaging of compact radio sources with milliarcsecs resolution. Use of phase-delay astrometric techniques in VLBI also allows determinations of relative positions with nearly microarcsecs accuracy (e.g., Marcaide & Shapiro 1983). Moreover, phase delay astrometry, unlike group-delay astrometry, permits a reliable subtraction of the source structure contribution to the delay, by referring such contribution to a reference point in the map. Such a (fixed) reference point provides an unambiguous identification of radio source features seen at different epochs, crucial for the determination of the absolute kinematics of the components (see Charlot et al. 1990; Guirado et al. 1995b; Ros et al. 1999).

In the past, our group has successfully applied phase-delay astrometry techniques to a number of pairs of radio sources. Guirado et al. (1995,1998) studied the pair QSO1928+738/BL2007+777 (about 5$^\circ$ apart) at 2.3, 5, and 8.4 GHz. Ros et al. (1999) added a new source (BL1803+784) to this pair, and extended the phase-connection technique (Shapiro et al. 1979) to separations of 6 $.\!\!^\circ$8 from observations at 8.4GHz. Pérez-Torres et al. (2000) successfully applied the phase-connection technique to the radio sources QSO1150+812/BL1803+784 (nearly 15$^\circ$ apart), from observations at 2.3 and 8.4GHz. In addition, Ros et al. (2000) and Pérez-Torres et al. (2000) have demonstrated that dual-frequency observations are not required anymore to subtract the ionospheric delay from the data. Indeed, those authors showed that it is possible to accurately model the ionosphere contribution from Global Positioning System data taken at sites nearby VLBI stations.

The astrometric results mentioned above have encouraged us to try to demonstrate the feasibility of the phase-connection technique for a complete sample of radio sources, using single-frequency VLBI observations, with the aim of studying the absolute kinematics of all the sources in the sample. Our sample consists of the 13 sources selected by Eckart et al. (1986,1987) from the S5 survey (Kühr et al. 1981), with the following selection criteria: a) declination $\delta\ge70^{\circ}$, galactic latitude $\vert b_{\rm II}\vert\ge10^{\circ}$; b) flux density $S_{\rm 5\,GHz}\ge1$ Jy at the epoch of the survey; and c) spectral index $\alpha_{\rm 2.7,\,5~GHz}\ge-0.5$ ( $S\sim\nu^{+\alpha}$). Throughout the paper, we will refer to those 13 radio sources as "the complete S5 polar cap sample".

We show in Fig. 1 the sky distribution of the members of the complete S5 polar cap sample, and indicate their names and relative angular separations. All 13 members of this sample are at such close angular distances that the use of an appropriate observing scheme permits a successful phase-connection of the data for a 24-hour observing Very Long Baseline Array (VLBA) run. Provided that enough phase-delay data (usually several hours) are acquired for each of the 13 radio sources, we should be able to solve for their positions with accuracies better than 0.1 milliarcsecs (mas). Once we precisely determine the relative positions of all the members of the S5 polar cap sample at 8.4 GHz for several epochs, the registration of the maps should be straightforward.

Such an astrometric analysis should allow us to unambiguously discern - for each of the 13 radio sources - which components are moving and which are stationary. Finding stationary components in 15 sources (the 13 of our sample, 3C345, Bartel et al. 1986; and 1038+528 A, Marcaide et al. 1994) would strongly support the standard model for AGNs (Blandford & Königl 1979). On the contrary, finding that some have no stationary components might reveal phenomena unthought. Knowing which components move and how, in an absolute sense, should allow us to propose the correct physical models.

As a first step in the astrometric process, we present here the hybrid maps of all radio sources at 8.4GHz for the two observing epochs. The astrometric results will be shown in further publications.

We concisely describe the 8.4GHz observations in Sect. 2, and present the source maps corresponding to the first (1997.93) and second (1999.41) epochs for all 13 radio sources in Sect. 3. We conclude with some summary remarks in Sect. 4.

Figure 1: Distribution of the complete S5 polar cap sample in the northern sky, centered at the celestial north pole. The black dots represent the positions of radio sources, and the angular distances among them are indicated in arc degrees with arrows.