Probing the Non-thermal Emission Geometry of AR Sco via Optical Phase-Resolved Polarimetry

TL;DR

This study applies optical polarimetric modeling to AR Sco, revealing stable, asymmetric emission regions and small particle pitch angles, advancing understanding of white dwarf pulsar emission geometry.

Contribution

It extends the rotating vector model to phase-resolved optical polarimetry of AR Sco, providing new insights into its emission geometry and magnetic inclination variations.

Findings

Detected ~10° variation in magnetic inclination angle α over orbit.

Observed ~30° variation in observer angle ζ over orbit.

Indicated emission originates close to the white dwarf surface.

Abstract

AR Sco is a binary system that contains a white and red dwarf. The rotation rate of the white dwarf has been observed to slow down, analogous to rotation-powered radio pulsars; it has thus been dubbed a "white dwarf pulsar". We previously fit the traditional radio pulsar rotating vector model to the linearly polarised optical data from this source, constraining the system geometry as well as the white dwarf mass. Using a much more extensive dataset, we now explore the application of the same model to binary phase-resolved optical polarimetric data, thought to be the result of non-thermal synchrotron radiation, and derive the magnetic inclination angle $\alpha$ and the observer angle $\zeta$ at different orbital phases. We obtain a $\sim 10^{\circ}$ variation in $\alpha$ and $\sim 30^{\circ}$ variation in $\zeta$ over the orbital period. The variation patterns in these two parameters is robust, regardless of the binning and epoch of data used. We speculate that the observer is detecting radiation from an asymmetric emission region that is a stable structure over several orbital periods. The success of this simple model lastly implies that the pitch angles of the particles are small and the pulsed, non-thermal emission originates relatively close to the white dwarf surface.