HST Observations of the Cataclysmic Variable Propeller LAMOST J024048.51+195226.9

We present Hubble Space Telescope (HST) far-UV spectra and light curves of the magnetic cataclysmic variable (CV) LAMOST J024048.51+195226.9 (J0240), the second known CV propeller. The five consecutive HST orbits span a full 7.34 hr binary orbital period. We detect a 24.939 ± 0.006 s far-UV modulation, confirming that J0240 contains the fastest spinning white dwarf (WD) in a CV. A high N v /C iv emission line ratio is considered an indicator of a recent episode of thermal-timescale mass transfer. The observed ratio in J0240 is higher than seen in typical magnetic CVs, but far less than observed in the only other confirmed propeller, AE Aquarii (AE Aqr). We also find that J0240 is significantly less luminous than AE Aqr during both low- and high-flux states. Around orbital phase 0.5, the Si iv emission line displays a P-Cygni absorption profile, likely related to the gas accelerated in the propeller. We derive new mass-dependent limits for the surface temperature of the WD of T ≤ 11,000–15,000 K. This temperature is low enough to allow for WD core crystallization, which may be linked to magnetism in WDs, particularly those in CVs.