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

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.

Polarimetric Evidence of the First White Dwarf Pulsar: The Binary System AR Scorpii

“…These comprise both optical lines (Balmer and He I) and UV lines (Si IV and He II). The discovery by Buckley et al [2] that AR Sco is highly linearly polarized (up to 40%) and, more importantly, that the polarization is strongly modulated (up to 90% pulse fraction) on both the spin and beat periods, has strengthened the evidence that AR Sco behaves as a pulsar. The high level of polarization also led Buckley et al [2] to conclude that the white dwarf is highly magnetic, with a field strength as high as 500 MG.…”

“…The discovery by Buckley et al [2] that AR Sco is highly linearly polarized (up to 40%) and, more importantly, that the polarization is strongly modulated (up to 90% pulse fraction) on both the spin and beat periods, has strengthened the evidence that AR Sco behaves as a pulsar. The high level of polarization also led Buckley et al [2] to conclude that the white dwarf is highly magnetic, with a field strength as high as 500 MG. Various interpretations have been proposed to explain the observed properties, including direct MHD interactions between the magnetic fields of both components [2], possibly producing bow shocks close to the surface of the M-star companion (e.g., [3,4]).…”

“…There is also the tell-tale absence of high frequency (>1 Hz) flickering associated with accretion in cataclysmic variables, in both magnetic and non-magnetic systems. Therefore the conclusion reached by [1,2] is that AR Sco is a detached binary, with no signs of accretion or mass loss through L1, powered predominantly by the spin-down of the magnetic white dwarf.…”

“…The conclusion (e.g., [1,2]) that the white dwarf has a high magnetic field strength raised the possibility that AR Sco was an asynchronous magnetic CV, or intermediate polar, which are typically quite hard (kT ∼ 10-20 keV) and moderately luminous X-ray sources (∼10 31 -10 33 erg s −1 ). In contrast, AR Sco has an X-ray luminosity of 5 ×10 30 erg s −1 (from a Swift observation [1]), inconsistent with it being an intermediate polar.…”

“…The high level of polarization also led Buckley et al [2] to conclude that the white dwarf is highly magnetic, with a field strength as high as 500 MG. Various interpretations have been proposed to explain the observed properties, including direct MHD interactions between the magnetic fields of both components [2], possibly producing bow shocks close to the surface of the M-star companion (e.g., [3,4]). …”