VLA Radio Observations of AR Sco

TL;DR

This study uses multi-frequency radio observations to analyze AR Sco's pulsations and polarization, revealing that its radio emission mainly originates near the M-dwarf star and involves cyclotron emission.

Contribution

First detailed multi-frequency radio imaging of AR Sco, demonstrating the origin of radio flux and polarization characteristics, and identifying cyclotron emission as a key component.

Findings

Radio flux modulates with orbital and beat periods.

Radio emission shows strong circular polarization (~30%).

Cyclotron emission likely dominates at low frequencies.

Abstract

AR Scorpii is unique amongst known white dwarf binaries in showing powerful pulsations extending to radio frequencies. Here we aim to investigate the multi-frequency radio emission of AR Sco in detail, in order to constrain its origin and emission mechanisms. We present interferometric radio frequency imaging of AR Sco at 1.5, 5 and 9 GHz, analysing the total flux and polarization behaviour of this source at high time resolution (10, 3 and 3 s), across a full 3.6 hr orbital period in each band. We find strong modulation of the radio flux on the orbital period and the orbital sideband of the white dwarf's spin period (also known as the "beat" period). This indicates that, like the optical flux, the radio flux arises predominantly from on or near the inner surface of the M-dwarf companion star. The beat-phase pulsations of AR Sco decrease in strength with decreasing frequency. They are strongest at 9 GHz and at an orbital phase ~0.5. Unlike the optical emission from this source, radio emission from AR Sco shows weak linear polarization but very strong circular polarization, reaching ~30% at an orbital phase ~0.8. We infer the probable existence of a non-relativistic cyclotron emission component, which dominates at low radio frequencies. Given the required magnetic fields, this also likely arises from on or near the M-dwarf.