VLBI astrometry on the white dwarf pulsar AR Scorpii

TL;DR

This study uses multi-epoch VLBI observations to precisely measure the astrometric parameters of AR Scorpii, confirming its distance and motion, and providing insights into its radio emission location within the white dwarf's light cylinder.

Contribution

It presents the first high-precision VLBI astrometry of AR Scorpii, independently confirming Gaia measurements and analyzing its kinematics and emission region.

Findings

Parallax of 8.52 mas, distance ~117 pc

Proper motion measurements consistent with Gaia

Radio emission likely within the white dwarf's light cylinder

Abstract

AR Scorpii (AR Sco), the only-known radio-pulsing white dwarf binary, shows unusual pulsating emission at the radio, infrared, optical and ultraviolet bands. To determine its astrometric parameters at the radio band independently, we conducted multi-epoch Very Long Baseline Interferometry (VLBI) phase-referencing observations with the European VLBI Network (EVN) at 5 GHz and the Chinese VLBI Network (CVN) plus the Warkworth 30-metre telescope (New Zealand) at 8.6 GHz. By using the differential VLBI astrometry, we provide high-precision astrometric measurements on the parallax ($\pi=8.52_{-0.07}^{+0.04}$ mas), and proper motion ($\mu_{\alpha}=9.48_{-0.07}^{+0.04}$ mas yr$^{-1}$, $\mu_{\delta}=-51.32_{-0.38}^{+0.22}$ mas yr$^{-1}$). The new VLBI results agree with the optical Gaia astrometry. Our kinematic analysis reveals that the Galactic space velocities of AR Sco are quite consistent with that of both intermediate polars (IPs) and polars. Combined with the previous tightest VLBI constraint on the size, our parallax distance suggests that the radio emission of AR Sco should be located within the light cylinder of its white dwarf.