A Model for AR~Scorpii: Emission from Relativistic Electrons Trapped by Closed Magnetic Field Lines of Magnetic White Dwarfs

Abstract

AR~Scorpii is an intermediate polar system composed of a magnetic white dwarf (WD) and an M-type star, and shows non-thermal, pulsed, and highly linearly polarized emission. The radio/optical emission modulates with the WD's spin and show the double peak structure in the light curves. In this paper, we discuss a possible scenario for the radiation mechanism of AR~Scorpii. The magnetic interaction on the surface of the companion star produces an outflow from the companion star, the heating of the companion star surface, and the acceleration of electrons to a relativistic energy. The accelerated electrons, whose typical Lorentz factor is $\sim 50-100$, from the companion star move along the magnetic field lines toward the WD surface. The electrons injected with the pitch angle of $\sin {\theta }_{p,0}>0.05$ are subject to the magnetic mirror effect and are trapped in the closed magnetic field line region.We find that the emission from the first magnetic mirror points mainly contributes to the observed pulsed emission and the formation of the double-peak structure in the light curve. For the inclined rotator, the pulse peak in the calculated light curve shifts the position in the spin phase, and a Fourier analysis exhibits a beat frequency feature, which are consistent with the optical/UV observations. The pulse profile also evolves with the orbital phase owing to the effect of the viewing geometry. The model also interprets the global features of the observed spectral energy distribution in radio to X-ray energy bands. We also discuss the curvature radiation and the inverse-Compton scattering process in the outer gap accelerator of the WD in AR Scorpii and discuss the possibility of the detection by future high-energy missions.

33 pages, 12 figures, accepted for publication in ApJ

References in corpus (5)

• Magnetic White Dwarfs
• New white dwarf stars in the Sloan Digital Sky Survey Data Release 10
• A radio pulsing white dwarf binary star
• Polarimetric evidence of a white dwarf pulsar in the binary system AR Scorpii
• Explaining Inverted Temperature Loops in the Quiet Solar Corona with Magnetohydrodynamic Wave Mode Conversion

Cited by in corpus (10)

• Driving the Beat: Time-Resolved Spectra of the White Dwarf Pulsar AR Scorpii
• Long-term photometric monitoring and spectroscopy of the white dwarf pulsar AR Scorpii
• An Improved Spin-Down Rate for the Proposed White-Dwarf Pulsar AR Scorpii
• Unveiling the white dwarf in J191213.72-441045.1 through ultraviolet observations
• 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
• Searching for cataclysmic variable stars in unidentified X-ray sources
• Modelling the broadband emission from the white dwarf binary system AR Scorpii
• An X-ray study for white dwarf binary AR Scorpii
• On the Polarized Nonthermal Emission from AR Scorpii