Driving the Beat: Time-Resolved Spectra of the White Dwarf Pulsar AR Scorpii

Abstract

We obtained high temporal resolution spectroscopy of the unusual binary system AR Sco covering nearly an orbit. The H$\alpha$ emission shows a complex line structure similar to that seen in some polars during quiescence. Such emission is thought to be due to long-lived prominences originating on the red dwarf. A difference between AR Sco and these other systems is that the white dwarf in AR Sco is rapidly spinning relative to the orbital period. "Slingshot" prominences stable at 3 to 5 stellar radii require surface magnetic fields between 100 and 500 G. This is comparable to the estimated WD magnetic field strength near the surface of the secondary. Our time-resolved spectra also show emission fluxes, line equivalent widths, and continuum color varying over the orbit and the beat/spin periods of the system. During much of the orbit, the optical spectral variations are consistent with synchrotron emission with the highest energy electrons cooling between pulses. On the time-scale of the beat/spin period we detect red and blue-shifted H$\alpha$ emission flashes that reach velocities of 700 km/s. Red-shifted Balmer emission flashes are correlated with the bright phases of the continuum beat pulses while blue-shifted flashes appear to prefer the time of minimum in the beat light curve. We propose that much of the energy generated in AR Sco comes from fast magnetic reconnection events occurring near the inward face of the secondary and we show that the energy generated by magnetic reconnection can account for the observed excess luminosity from the system.

Accepted for publication in ApJ, December 27, 2018, 15 page, 14 figures

References in corpus (7)

• Chromospheric Anemone Jets as Evidence of Ubiquitous Reconnection
• A radio pulsing white dwarf binary star
• 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
• New Complexities in the Low-State line profiles of AM Herculis
• Modeling properties of chromospheric evaporation driven by thermal conduction fronts from reconnection shocks

Cited by in corpus (13)

• 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
• Confirmation of a Second Propeller: A High-Inclination Twin of AE~Aquarii
• Radio detections of two unusual cataclysmic variables in the VLA Sky Survey
• Unveiling the white dwarf in J191213.72-441045.1 through ultraviolet observations
• Peeking Between the Pulses: The Far-UV Spectrum of the Previously Unseen White Dwarf in AR Scorpii
• 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
• Properties of white dwarf in the binary system AR Scorpii and its observed features
• Towards Modelling AR Sco: Generalised Particle Dynamics and Strong Radiation-Reaction Regimes
• VLBI astrometry on the white dwarf pulsar AR Scorpii
• 21 years of Astronomy at Warwick: celebrating the legacy of Prof. Tom Marsh