Found: a rapidly spinning white dwarf in LAMOST J024048.51+195226.9

• Published in: Monthly notices of the Royal Astronomical Society. Letters 2021-11, Vol.509 (1), p.L31-L36
• Main Authors: Pelisoli, Ingrid, Marsh, T R, Dhillon, V S, Breedt, E, Brown, A J, Dyer, M J, Green, M J, Kerry, P, Littlefair, S P, Parsons, S G, Sahman, D I, Wild, J F
• Format: Article
• Language: English
• ISSN: 1745-3925; 1745-3933
• DOI: 10.1093/mnrasl/slab116

ABSTRACT We present optical photometry of the cataclysmic variable LAMOST J024048.51+195226.9 taken with the high-speed, five-band CCD camera HiPERCAM on the 10.4 m Gran Telescopio Canarias. We detect pulsations originating from the spin of its white dwarf, finding a spin period of 24.9328(38) s. The pulse amplitude is of the order of 0.2 per cent in the g band, below the detection limits of previous searches. This detection establishes LAMOST J024048.51+195226.9 as only the second white dwarf magnetic propeller system, a twin of its long-known predecessor, AE Aquarii. At 24.93 s, the white dwarf in LAMOST J024048.51+195226.9 has the shortest known spin period of any cataclysmic variable star. The white dwarf must have a mass of at least 0.7 M⊙ to sustain so short a period. The observed faintest u-band magnitude sets an upper limit on the white dwarf’s temperature of $\sim\!25\, 000$ K. The pulsation amplitudes measured in the five HiPERCAM filters are consistent with an accretion spot of $\sim\!30\, 000\,$K covering ∼2 per cent of the white dwarf’s visible area, although spots that are hot and smaller, or cooler and larger cannot be ruled out.