Constraining the Emission Geometry and Mass of the White Dwarf Pulsar AR Sco using the Rotating Vector Model

Abstract

We apply the standard radio pulsar rotating vector model to the white dwarf pulsar AR Sco's optical polarization position angle swings folded at the white dwarf's spin period as obtained by Buckley et al. (2017). Owing to the long duty cycle of spin pulsations with a good signal-to-noise ratio over the entire spin phase, in contrast to neutron star radio pulsars, we find well-constrained values for the magnetic obliquity $\alpha$ and observer viewing direction $\zeta$ with respect to the spin axis. We find $\cos \alpha =0.060_{-0.053}^{+0.050}$ and $\cos \zeta =0.49_{-0.08}^{+0.09}$, implying an orthogonal rotator with an observer angle $\zeta ={60.4^{\circ }}_{-6.0^{\circ }}^{+5.3^{\circ }}$. This orthogonal nature of the rotator is consistent with the optical light curve consisting of two pulses per spin period, separated by $180^{\circ }$ in phase. Under the assumption that $\zeta \approx i$, where $i$ is the orbital inclination, and a Roche-lobe-filling companion M star, we obtain $m_{\mathrm{WD}}=1.00_{-0.10}^{+0.16}{M}_{\odot }$ for the white dwarf mass. These polarization modeling results suggest the that non-thermal emission arises from a dipolar white dwarf magnetosphere and close to the star, with synchrotron radiation (if non-zero pitch angles can be maintained) being the plausible loss mechanism, marking AR Sco as an exceptional system for future theoretical and observational study.

11 pages, 7 figures, accepted by ApJ

References in corpus (7)

• A radio pulsing white dwarf binary star
• The spin axes orbital alignment of both stars within the eclipsing binary system V1143Cyg using the Rossiter-McLaughlin effect
• Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii
• AR Sco: A White Dwarf Synchronar
• A Model for AR~Scorpii: Emission from Relativistic Electrons Trapped by Closed Magnetic Field Lines of Magnetic White Dwarfs
• On the Polarized Nonthermal Emission from AR Scorpii
• Modelling the polarisation signatures detected from the first white dwarf pulsar AR Sco

Cited by in corpus (6)

• Long-term photometric monitoring and spectroscopy of the white dwarf pulsar AR Scorpii
• Probing the Non-thermal Emission Geometry of AR Sco via Optical Phase-Resolved Polarimetry
• Magnetospheric interaction in white dwarf binaries AR Sco and AE Aqr
• An X-ray study for white dwarf binary AR Scorpii
• VLBI astrometry on the white dwarf pulsar AR Scorpii
• Towards Modelling AR Sco: Generalised Particle Dynamics and Strong Radiation-Reaction Regimes