The study of compact radio sources with Very Long Baseline Interferometry (VLBI) has revealed some intrinsic properties of active galactic nuclei (AGN). In some cases, radio emission variability and structural changes relate to high energy outbursts in X-ray and/or -ray regimes. Current models suggest that the emission in these very different energy ranges is connected by means of Inverse Compton processes. This mechanism considers that the relativistic electrons, producing synchrotron radiation, upscatter low energy photons coming either from the synchrotron emission or from other sources (Maraschi et al. 1992).
The phenomenon of apparent superluminal motion, predicted by the theory and found by the VLBI observations, has been explained in terms of relativistic bulk motion along (or very close to) the line of sight of the emitting plasma in AGN. The understanding of the superluminal radio source phenomenon is important for a number of reasons. On one hand, the study of individual sources, possibly observed in a wide range of frequencies, is crucial to understand the emission mechanisms in various parts of the electromagnetic spectrum. On the other hand, the statistical analysis of large samples of superluminal sources addresses more general issues, such as, for example, intrinsic differences among the classes of optical counterparts of superluminal radio sources and tests on cosmological models (Vermeulen 1996). Statistical studies carried out thus far are based on a sample of 80 sources, mostly located in the Northern Hemisphere, and the extension of such the sample is important for improving of the statistics.
Two VLBI imaging surveys were carried out in the mid-southern hemisphere using MK2 VLBI recording system with the SHEVE (Southern Hemisphere VLBI Experiment) array plus Shanghai radio telescope at 5 GHz in November 1992 (Shen et al. 1997) and in May 1993 (Shen et al. 1998). The major aims of the experiments are a) search for new superluminal sources; b) fill the gap in southern VLBI observations; c) search for potential targets for Space VLBI; and d) identify compact radio sources as potential southern calibrators and astrometric reference sources. The observations provided first epoch images of 44 sources, and some superluminal candidates were found.
In order to monitor the most interesting sources found in the two previous surveys, and to get information on those radio sources at low declination which had not been observed with VLBI as yet, a third MK2 VLBI experiment at 5 GHz of sources located in the mid-southern hemisphere was planned and carried out, using Noto, Shanghai, Urumqi, and Hartebeesthoek telescopes in October 1995. In this paper we present the results of this third MK2 VLBI experiment. In Sect. 2 we describe the source sample. In Sect. 3 we describe the observations and data reduction. Section 4 is a summary of the results.
Throughout this paper, the values H0=100 h km s-1 Mpc-1, q0=0.5, and will be used.
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