Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii (1612.03185v1)

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 a White Dwarf Pulsar in the Binary System AR Scorpii

The paper by Buckley et al. presents a comprehensive investigation into the binary system AR Scorpii, identifying it as a unique case of a white dwarf pulsar. The system consists of a rapidly spinning white dwarf and a low-mass M-dwarf in a 3.56-hour orbit. Recent polarimetric observations provided evidence for strong linear polarization of the emitted light, reaching up to 40%, with variations synchronized to the spin and beat periods of the white dwarf. This discovery parallels certain characteristics typical of neutron star pulsars and suggests that the emission mechanics are driven by the spin-down energy loss of the white dwarf.

Results and Observations

Key findings demonstrate that the polarized radiation originates from synchrotron emission facilitated by magnetic interactions between the white dwarf and its M-star companion. Distinct modulation of the light's polarization occurs at spin and beat periods, indicating complex waveforms affected by magnetic interactions and relativistic beaming effects. The observations yield a highly polarized signal comparable to the pulsar emissions from systems like the Crab Nebula, albeit modulated by the binary interaction dynamics inherent to AR Scorpii.

Photopolarimetry

The photopolarimetric data acquired using the HIPPO polarimeter reveal significant modulations in linear polarization synchronized with the white dwarf's spin period. Analysis of Stokes parameters across consecutive nights showcases variability arising from phase and interaction changes within the binary cycle. These polarimetric observations substantiate the generation of synchrotron radiation through regular magnetic field alignment and reconnection events.

Periodic Modulations and SED

Through Fourier transform analysis, the paper outlines the power distribution across spin harmonics, noting compelling evidence for power at spin and beat frequencies. The extracted modulation patterns correlate with models proposing bipolar synchrotron outflows, facilitated by the magnetic poles of the white dwarf sweeping across the system's emission region. The SED splits into low-frequency synchrotron emission and higher-frequency components, indicating complex interactions within both stellar magnetospheres.

Implications and Future Directions

The research posits AR Scorpii as the first identified white dwarf pulsar, providing a framework for understanding magnetic interactions in such binaries. Beyond contributing to the compendium of stellar phenomena, this work urges deeper inquiry into magnetosphere dynamics across stellar types. The strong polarimetric signatures suggest potential advancements in modeling magnetic field interactions and energy dissipation processes in compact binary systems.

Future observational campaigns targeting radio and X-ray wavelengths are critical to refine understanding of binary interaction dynamics and synchrotron radiation mechanisms. Detailed polarimetric studies will further elucidate the geometry of the emission processes and confirm hypotheses surrounding pulsar phenomena in white dwarfs.

In conclusion, this research delineates a pivotal understanding of the binary system AR Scorpii and its implications for stellar magnetism and pulsar mechanics, thereby fostering opportunities for addressing the broader astrophysical principles governing compact, strongly magnetized binary systems.