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

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 (GTC). 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% 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 ∼ 25 000 K. The pulsation amplitudes measured in the five HiPERCAM filters are consistent with an accretion spot of ∼ 30 000 K covering ∼ 2% of the white dwarf's visible area, although much hotter and smaller spots cannot be ruled out.

The white dwarf binary merger model of GRB 170817A

“…Indeed, WDs of parameters approaching the present ones have been already identified, e.g. the WD in V1460 Her with P ≈ 39 s, 68 and the most recent observation of the WD in LAMOST J024048.51+195226.9 with P ≈ 25 s. 69 WDs of similar properties have been proposed as a model of SGRs and AXPs. [70][71][72][73] Therefore, the newborn WD pulsar in GRB 170817A could show itself in the near future as an SGR/AXP in the GRB 170817A sky position.…”