Time series photopolarimetry and modelling of the white dwarf pulsar in AR Scorpii

Abstract

We present detailed optical photo-polarimetric observations of the recently-discovered white dwarf pulsar AR Scorpii. Our extensive dataset reveals that the polarized emission is remarkably stable and repeatable with spin, beat and orbital modulations. This has enabled us to construct a simple geometrical model which assumes that all of the optically polarized emission emanates from two diametrically opposed synchrotron emission regions on the white dwarf magnetosphere. We suggest that the observed polarimetric modulations occur as a result of an enhanced injection of relativistic electrons into the magnetosphere of the white dwarf as it sweeps past the M-dwarf. This leads to an increase in synchrotron emission as the injected electrons accelerate towards each magnetic mirror point close to the magnetic poles of the white dwarf. Whilst this scenario reproduces the detailed polarimetric modulations, other suggested scenarios involving emission sites locked in the white dwarf rotating frame are not ruled out. For example, pulsar-like particle acceleration as a result of either electric potentials between the white dwarf and the light cylinder or a striped relativistic magnetohydrodynamic wind, outside the light cylinder. Previous conclusions that argued that the observed strong optical beat modulations require that the optical polarization arises predominantly from or near the M-dwarf are inconsistent with our observations.