On the coherence loss in phase-referenced VLBI observations

TL;DR

This paper investigates how atmospheric effects and calibrator-target separation impact the coherence and image quality in phase-referenced VLBI observations, revealing limitations on dynamic range and flux accuracy.

Contribution

It provides a systematic analysis of signal decoherence in VLBI phase referencing across frequencies and separations, with empirical data and comparison to existing models.

Findings

Atmospheric effects limit the dynamic range at all frequencies.

Dynamic range decreases inversely with the sine of the separation angle.

Peak flux densities decline with increasing calibrator-target separation.

Abstract

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.