HST Observations of the CV Propeller LAMOST J024048.51+195226.9

TL;DR

This paper presents HST observations of the magnetic CV J0240, revealing the fastest-spinning white dwarf, unique emission features, and insights into its temperature and accretion behavior, advancing understanding of CV propellers.

Contribution

First detailed HST FUV spectra and light curves of J0240, confirming its status as the fastest-spinning white dwarf in a CV and providing new temperature constraints.

Findings

Detected 24.939 s FUV modulation confirming fastest-spinning WD

Observed high N V/C IV ratio indicating recent thermal mass transfer

Derived WD surface temperature limits of 11,000-15,000 K

Abstract

We present Hubble Space Telescope (HST) FUV 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 hour binary orbital period. We detect a 24.939 $\pm$ 0.006 s FUV 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 time-scale 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 temperature limits for the surface temperature of the WD of T $\leq$ 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.