High-time-resolution Photometry of AR Scorpii: Confirmation of the White Dwarf’s Spin-down

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 ofν = (−5.14 ± 0.32) × 10 −17 Hz s −1 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.

White Dwarf Pulsars as Possible Gamma-Ray Sources

“…Initial pulse timing studies after its discovery, over a baseline of 7 years, suggest that the white dwarf is slowing down, resulting in a frequency derivative of -ν s,wd = 2.86 × 10 −17 Hz s −1 , which for a ∼ 0.8 M ⊙ white dwarf (R wd ≈ 0.01R ⊙ ), infer a spin-down luminosity of approximately Lν s,wd ≈ 1.5 × 10 33 erg s −1 [1]). The result of a more recent study of the spin-evolution of the white dwarf [7] is inconsistent with any reported spin-down, while another study [8] shows an evidence of a spin-down rate through a beat frequency derivative -ν s,b = 5.14 × 10 −17 Hz s −1 with resultant spin-down power of Lν s,wd ≈ 3 × 10 33 erg s −1 which is approximately twice the value reported earlier [1].…”

Confirmation of a Second Propeller: A High-inclination Twin of AE Aquarii

“…J0240 appears to be only the second member of the AE Aqr subclass of IPs. The proposed WD pulsar, AR Sco (Marsh et al 2016;Stiller et al 2018;Garnavich et al 2019) may also be related to these AE Aqr-like propellers. The propellers and AR Sco appear to efficiently extract spin energy from their WDs, and are unusually strong radio emitters, implying a significant nonthermal power source derived from an interaction with the rapidly spinning magnetic WD.…”

V1460 Her: a fast spinning white dwarf accreting from an evolved donor star

“…Not only does it have a clearly evolved donor star, but it contains a rapidly spinning white dwarf, which is only a little slower than the white dwarf in AE Aqr itself. It will be fascinating to see whether, like AE Aqr and the possibly-related system AR Sco (Marsh et al 2016;Stiller et al 2018), the white dwarf in V1460 Her is in a state of rapid spin-down. This will require alias-free measurement of the pulsation phase over a number of years, not possible from the data we have in hand at present.…”