On the Polarized Nonthermal Emission from AR Scorpii

Abstract

We study linear polarization of optical emission from white dwarf (WD) binary system AR~Scorpii. The optical emission from this binary is modulating with the beat frequency of the system, and it is highly polarized, with the degree of the polarization reaching $\sim 40$\%. The angle of the polarization monotonically increases with the spin phase, and the total swing angle can reach $360^{\circ }$ over one spin phase. It is also observed that the morphology of the pulse profile and the degree of linear polarization evolve with the orbital phase. These polarization properties can constrain the scenario for nonthermal emission from AR Scorpii. In this paper, we study the polarization properties predicted by the emission model, in which (i) the pulsed optical emission is produced by the synchrotron emission from relativistic electrons trapped by magnetic field lines of the WD and (ii) the emission is mainly produced at magnetic mirror points of the electron motion. We find that this model can reproduce the large swing of the polarization angle, provided that the distribution of the initial pitch angle of the electrons that are leaving the M-type star is biased to a smaller angle rather than a uniform distribution. The observed direction of the swing suggests that the Earth viewing angle is less than $90^{\circ }$ measured from the WD spin axis. The current model prefers an Earth viewing angle of $50^{\circ }-60^{\circ }$ and a magnetic inclination angle of $50^{\circ }-60^{\circ }$ (or $120^{\circ }-130^{\circ }$). We discuss that the different contribution of the emission from M-type star to total emission causes a large variation in the pulsed fraction and the degree of the linear polarization along the orbital phase.

11 pages, 9 figures. accepted for publication in ApJ

References in corpus (7)

• Magnetic White Dwarfs
• A radio pulsing white dwarf binary star
• Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii
• Pulse Profiles, Spectra and Polarization Characteristics of Non-Thermal Emissions from the Crab-Like Pulsars
• 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
• Long-term photometric variations in the candidate white-dwarf pulsar AR Scorpii from K2, CRTS, and ASAS-SN observations

Cited by in corpus (4)

• Constraining the Emission Geometry and Mass of the White Dwarf Pulsar AR Sco using the Rotating Vector Model
• Probing the Non-thermal Emission Geometry of AR Sco via Optical Phase-Resolved Polarimetry
• Modelling the broadband emission from the white dwarf binary system AR Scorpii
• An X-ray study for white dwarf binary AR Scorpii