Since its discovery in 1985 (cf. Witzel et al. 1986; Heeschen et al. 1987) it has been shown that intraday variability (IDV, i.e., flux-density variations on time scales less than a day) is a common, but complex phenomenon among compact flat-spectrum extragalactic radio sources. It is detected in a large fraction of this class of sources (e.g. Quirrenbach et al. 1992). Variations with amplitudes of up to a factor of two have been reported in the radio as well as optical bands (Quirrenbach et al. 1992; Wagner & Witzel 1995; Kedziora-Chudczer et al. 1997). The occurrence of IDV appears to be correlated with the milliarcsecond structure of the sources: it is very common in objects dominated by a compact VLBI core, but is not normally found in sources with a prominent VLBI jet (Quirrenbach et al. 1992). In parallel to the variability of the total flux density, variations of the linearly polarized flux density and the polarization angle have been observed in many sources (Quirrenbach et al. 1989b, Kraus et al. 1999a,c; Kraus et al., in preparation). Both correlations (e.g. 0716+714) and anti-correlations (e.g. 0917+624) between the total and the polarized flux density have been seen (Wegner 1994). In 0716+714 and 0954+658 there is evidence for a correlation between the rapid flux density variability in the radio and the optical regime (Quirrenbach et al. 1991; Wagner et al. 1993). Such an effect would severely constrain any models of IDV and rule out propagation effects as their only cause. From light travel time arguments, it is clear that IDV gives insight in very small parts of the AGN (light days to light hours), if it is intrinsic to the sources. In that case it implies apparent brightness temperatures of K to K, far in excess of the inverse Compton limit of K (Kellermann & Pauliny-Toth 1969).
In this paper we present the results of three observing campaigns carried out with the Very Large Array in order to study intraday variations in a number of compact radio sources. We point out that these sources were selected mainly on the basis of noticeable variations in earlier observations with the Effelsberg 100m telescope; they do not form a statistically complete sample in any sense. (For an analysis of IDV in statistically complete samples see Quirrenbach et al. 1992.) In the next section the observations and data reduction procedures are described, afterwards the methods of statistical analysis are introduced. The properties of all program sources are listed in Sect. 5. All observations are summarized in tabular form, and for all program sources the light curves are plotted. In addition, for the sources which showed IDV in total intensity, we present the structure functions.
Additional observations of intraday variability performed with the Effelsberg 100m telescope, and a more detailed interpretation of the available data on intraday radio variability will be presented in forthcoming Papers II and III in this series (Kraus et al., in preparation).
Subsets of the data presented in here have already been published (Quirrenbach et al. 1989b; Qian et al. 1991; Wagner et al. 1996; Kraus et al. 1999b). In addition to the observations of extragalactic sources, two radio stars (UXAri, V711Tau) were included in the October 1992 observing run (Elias et al. 1995).
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