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 24.93s, 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 0.7MSun to sustain so short a period. The observed faintest u-band magnitude sets an upper limit on the white dwarf's temperature of ~25000K. The pulsation amplitudes measured in the five HiPERCAM filters are consistent with an accretion spot of ~30000K covering ~2% of the white dwarf's visible area, although spots that are hot and smaller, or cooler and larger cannot be ruled out.

• The paper reports the first detection of pulsations, establishing a 24.93-second spin period in the white dwarf of J0240+1952.
• It employs high-speed, multi-band photometry using HiPERCAM on the 10.4 m GTC to constrain key parameters, including a minimum mass of 0.7 M☉ and a temperature near 25,000 K.
• The analysis identifies an accretion spot (~30,000 K covering 2% of the surface), clarifying the magnetic propeller mechanism in cataclysmic variables.

Rapid Spin of the White Dwarf in LAMOST J024048.51+195226.9

The paper presents an observational study of the cataclysmic variable LAMOST J024048.51+195226.9 (hereafter J0240+1952) focusing on the spin characteristics of its white dwarf component. The research employs high-speed, multi-band photometry using the HiPERCAM facility on the 10.4 m Gran Telescopio Canarias (GTC). This investigation marks the first detection of pulsations emanating from the spin of the white dwarf in J0240+1952, affirming a spin period of approximately 24.933 seconds. Notably, this discovery positions J0240+1952 as the second system classified as a white dwarf magnetic propeller, sharing this distinction with the previously singular system, AE Aquarii (AE Aqr).

Key Findings and Analysis

• Spin Period and Amplitude: J0240+1952's white dwarf has a spin period of 24.93 seconds, making it the fastest known spin for any white dwarf in cataclysmic variables. The amplitude of the pulse in the g-band was measured at approximately 0.23%, underscoring the hypersensitivity required for detection, a level below the detection capability of prior searches.
• White Dwarf Characteristics: Calculations infer that the white dwarf in J0240+1952 must have a mass of at least 0.7 $\mathrm{M}_{\odot}$ to sustain such a rapid spin rate without structural instability. The upper limit on the white dwarf's temperature is derived to be around 25,000 K, based on the faintest observed u-band magnitude. These constraints on temperature and mass align with expectations for cataclysmic variable systems, enhancing the characterisation of rapidly spinning white dwarfs within these environments.
• Accretion Spot Analysis: The observed pulsation amplitudes across five HiPERCAM filters suggest the presence of an accretion spot with a temperature near 30,000 K, covering approximately 2% of the visible area of the white dwarf. Alternative configurations such as smaller, hotter spots or larger, cooler spots could also potentially describe the data, indicating a necessity for further observations at different wavelengths to refine these models.

Implications and Future Directions

The confirmation of J0240+1952 as a white dwarf magnetic propeller system similar to AE Aqr considerably enriches the framework for understanding such systems. It elucidates the scenario where the accreted matter is expelled from the system due to interactions with the white dwarf's intense magnetosphere, rather than accreting directly, resulting in distinguished observational features.

From a theoretical perspective, the presence of another propeller system provides a broader context for examining the mechanisms and evolution of binary systems dominated by magnetic interactions. Practically, the work suggests pathways for future observational campaigns that could refine spin period measurements, study the long-term spin-down behavior, and quantify the expulsion efficiency of accretion material in magnetic propellers.

In conclusion, this study advances the understanding of rapid spin phenomena in cataclysmic variable stars while opening new questions about the dynamics of accretion and the role of magnetic fields in binary star evolution. This work also emphasizes the necessity of high-temporal-resolution observations in unveiling the fast-paced phenomena associated with white dwarf stars. Future investigations may include detailed spectroscopic follow-ups, leveraging more sensitive instrumentation to explore these systems' stellar and magnetic properties more deeply.