Abstract
ABSTRACT Accreting magnetic white dwarfs offer an opportunity to understand the interplay between spin-up and spin-down torques in binary systems. Monitoring of the white dwarf spin may reveal whether the white dwarf spin is currently in a state of near-equilibrium, or of unidirectional evolution towards longer or shorter periods, reflecting the recent history of the system and providing constraints for evolutionary models. This makes the monitoring of the spin history of magnetic white dwarfs of high interest. In this paper, we report the results of a campaign of follow-up optical photometry to detect and track the 39- s white dwarf spin pulses recently discovered in Hubble Space Telescope data of the cataclysmic variable V1460 Her. We find the spin pulsations to be present in the g-band photometry at a typical amplitude of 0.4 per cent. Under favourable observing conditions, the spin signal is detectable using 2-m class telescopes. We measured pulse-arrival times for all our observations, which allowed us to derive a precise ephemeris for the white dwarf spin. We have also derived an orbital modulation correction that can be applied to the measurements. With our limited baseline of just over 4 yr, we detect no evidence yet for spin-up or spin-down of the white dwarf, obtaining a lower limit of $|P/\dot{P}| \gt 4\times 10^{7}$ yr, which is already four to eight times longer than the time-scales measured in two other cataclysmic variable systems containing rapidly rotating white dwarfs, AE Aqr and AR Sco.
Highlights
The spin rates of accreting white dwarfs in cataclysmic variable binary stars are determined by competing mechanisms, which add or extract angular momentum
Magnetic white dwarfs are of particular interest because they reveal their spin periods through photometric variations induced by spots, which result from accretion rate asymmetries caused by their fields
We present the results of this campaign aimed at establishing the spin history of the magnetic white dwarf V1460 Her since its recent discovery
Summary
The spin rates of accreting white dwarfs in cataclysmic variable binary stars are determined by competing mechanisms, which add or extract angular momentum. The white dwarf in AR Sco, whose spin period is 118 s, is spinning down on a timescale of 5 × 106 yr (Stiller et al 2018; Gaibor et al 2020), and in this case the system appears to be entirely detached (Marsh et al 2016; Garnavich et al 2021a), with no accretion and no evidence for propeller-induced flaring activity (Littlefield et al 2017) It is not yet clear how systems like AE Aqr and AR Sco achieved their current states. Schreiber et al.’s (2021) hypothesis makes accreting white dwarfs of short spin period of very high interest, as they could be systems whose fields have only recently emerged, and they might be in a state of rapid spin-down Whether this is the case, or whether they are in a state of quasi-equilibrium, possible for relatively weak fields at short spin periods, can only be established through observation. We present the results of this campaign aimed at establishing the spin history of the magnetic white dwarf V1460 Her since its recent discovery
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