Independent VLBI astrometric results to validate the Gaia results for AR Sco, the only known radio-pulsing white dwarf

VLBI astrometric observations of radio stars can validate the quality of Gaia Celestial Reference Frame (GCRF) and help to improve the accuracy and robustness of the link between the International Celestial Reference Frame (ICRF) and GCRF. AR Scorpii (AR Sco) is the only known radio-pulsing white dwarf binary to date, consisting of a rapidly rotating magnetic white dwarf and a M-type main-sequence star. It has a broadband spectrum and unusual pulsations detected at the radio, infrared, optical, and ultraviolet bands.

Recently, an international research team led by Dr. Lang Cui from the Xinjiang Astronomical Observatory, Chinese Academy of Sciences, successfully imaged the compact weak radio emission region and measured the astrometric parameters with high precision for the radio star AR Sco, by using multi-epoch VLBI observations. The results have been published in the Monthly Notices of the Royal Astronomical Society (MNRAS).

To determine the astrometric parameters of AR Sco at radio band independently, the researchers conducted multi-epoch VLBI phase-referencing observations with the European VLBI Network (EVN) and the Chinese VLBI Network (CVN) plus the New Zealand Warkworth 30-m telescope. Besides the primary calibrator, an additional weak extragalactic source very closing to the target AR Sco, played a key role as secondary calibrator for phase-referencing to improve the astrometric precision when data reduction. Finally, the researchers clearly detected the compact radio emission (Fig. 1) and provided high-precision astrometric measurements (Fig. 2) for AR Sco. This work provides new and independent astrometric results to validate the Gaia results for AR Sco.

In addition, the researchers estimated the upper limit of the radio emitting region size of AR Sco and suggested that the radio emission should be located within the light cylinder of its white dwarf. Furthermore, based on the astrometric results, they analyzed the kinematics of AR Sco and found that the Galactic space velocities of AR Sco are quite consistent with that of both intermediate polars (IPs) and polars.

Published in Jiang, P.-F.; Cui, L.; Yang, J. et al., 2023, MNRAS, 520, 2942