Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii

TL;DR

AR Scorpii exhibits pulsar-like behavior powered by a highly magnetized, rapidly rotating white dwarf, with strong, periodic polarization signals indicating magnetic interactions and synchrotron emission, marking it as the first white dwarf pulsar.

Contribution

This study provides the first evidence of a white dwarf pulsar, demonstrating pulsar-like emission mechanisms in a white dwarf system through polarization observations.

Findings

Strong linear polarization up to 40% observed

Polarization varies periodically with spin and beat periods

Supports white dwarf spin-down powering pulsar-like emission

Abstract

The variable star AR Sco was recently discovered to pulse in brightness every 1.97 min from ultraviolet wavelengths into the radio regime. The system is composed of a cool, low-mass star in a tight, 3.55 hr orbit with a more massive white dwarf. Here we report new optical observations of AR Sco that show strong linear polarization (up to 40%) which varies strongly and periodically on both the spin period of the white dwarf and the beat period between the spin and orbital period, as well as low level (< a few %) circular polarization. These observations support the notion that, similar to neutron star pulsars, the pulsed luminosity of AR Sco is powered by the spin-down of the rapidly-rotating white dwarf which is highly magnetised (up to 500 MG). The morphology of the modulated linear polarization is similar to that seen in the Crab pulsar, albeit with a more complex waveform owing to the presence of two periodic signals of similar frequency. Magnetic interactions between the two component stars, coupled with synchrotron radiation from the white dwarf, power the observed polarized and non-polarized emission. AR Scorpii is therefore the first example of a white dwarf pulsar.