High-Time-Resolution Photometry of AR Scorpii: Confirmation of the White Dwarf's Spin-Down

TL;DR

This study provides detailed, high-time-resolution optical photometry of AR Scorpii, confirming the white dwarf's spin-down and elucidating the interplay of pulsations and orbital variations in this unique binary system.

Contribution

The paper offers the first detailed confirmation of the white dwarf's spin-down in AR Scorpii through precise timing measurements and models the complex pulsation interactions.

Findings

Confirmed the white dwarf's spin-down with a measured rate of -5.14 x 10^-17 Hz/s.

Showed that pulsation variations are due to interference between beat and spin signals.

Demonstrated that the white dwarf's rotational energy powers the system's electromagnetic emission.

Abstract

The unique binary AR Scorpii consists of an asynchronously rotating, magnetized white dwarf (WD) that interacts with its red-dwarf companion to produce a large-amplitude, highly coherent pulsation every 1.97 minutes. Over the course of two years, we obtained thirty-nine hours of time-resolved, optical photometry of AR Sco at a typical cadence of 5 seconds to study this pulsation. We find that it undergoes significant changes across the binary orbital period and that its amplitude, phase, and waveform all vary as a function of orbital phase. We show that these variations can be explained by constructive and destructive interference between two periodic, double-peaked signals: the spin-orbit beat pulse, and a weaker WD spin pulse. Modelling of the light curve indicates that in the optical, the amplitude of the primary spin pulse is 50% of the primary beat amplitude, while the secondary maxima of the beat and spin pulses have similar amplitudes. Finally, we use our timings of the beat pulses to confirm the presence of the disputed spin-down of the WD. We measure a beat-frequency derivative of -5.14(32) x 10^-17 Hz/s and show that this is attributable to the spin-down of the WD. This value is approximately twice as large as the estimate from Marsh et al. (2016) but is nevertheless consistent with the constraints established in Potter & Buckley (2018). Our precise measurement of the spin-down rate confirms that the decaying rotational energy of the magnetized white dwarf is sufficient to power the excess electromagnetic radiation emitted by the binary.