Context: Phase referencing is a standard calibration technique in radio
interferometry, particularly suited for the detection of weak sources close to
the sensitivity limits of the interferometers. However, effects from a changing
atmosphere and inaccuracies in the correlator model may affect the
phase-referenced images, leading to wrong estimates of source flux densities
and positions. A systematic observational study of signal decoherence in phase
referencing, and its effects in the image plane, has not been performed yet.
Aims: We systematically studied how the signal coherence in
Very-Long-Baseline-Interferometry (VLBI) observations is affected by a
phase-reference calibration at different frequencies and for different
calibrator-to-target separations. The results obtained should be of interest
for a correct interpretation of many phase-referenced observations with VLBI.
Methods: We observed a set of 13 strong sources (the S5 polar cap sample) at
8.4 and 15 GHz in phase-reference mode, with 32 different calibrator/target
combinations spanning angular separations between 1.5 and 20.5 degrees. We
obtained phase-referenced images and studied how the dynamic range and peak
flux density depend on observing frequency and source separation.
Results: We obtained dynamic ranges and peak flux densities of the
phase-referenced images as a function of frequency and separation from the
calibrator. We compared our results with models and phenomenological equations
previously reported.
Conclusions: The dynamic range of the phase-referenced images is strongly
limited by the atmosphere at all frequencies and for all source separations.
The limiting dynamic range is inversely proportional to the sine of the
calibrator-to-target separation. We also find that the peak flux densities,
relative to those obtained with the self-calibrated images, decrease with
source separation.Comment: 5 pages, 3 figures. Submitted to A&A on 5.02.2010; accepted on
11.03.2010