Discovery of a Nearby Ultra-fast Magnetic White Dwarf Propeller: Common Pathways to Magnetic Cataclysmic Variables

The fastest spinning white dwarfs (WDs), which approach break-up speeds, are found in close binary systems. In the two known magnetic WD propellers'', AE~Aqr and LAMOST J0240+1952, the magnetic WD spins so rapidly that it ejects the material transferred by its Roche-lobe-filling companion rather than accreting it. We report the discovery of the third such system, 1eRASS J042633.5$-$250226 (1eRASS J0426/Valeria), at only 265 pc, identified by combining SRG/eROSITA and \textit{Gaia} data via the X-ray Main Sequence’'. TESS photometry shows AE~Aqr-like flaring and orbital modulation with $P_{\mathrm{o}\mathrm{r}\mathrm{b}}\approx 6.62$ hr, and ZTF shows month-long high/low state changes. Phase-resolved Palomar/DBSP spectroscopy confirms the orbital period and yields a donor radial velocity of $K_2=202\pm 7$ km s${}^{-1}$. High-speed Palomar/CHIMERA photometry detects optical pulsations on three nights with the strongest detection at $P_{\text{spin}}=26.8$ sec, though further observations are needed to confirm this as the WD spin. Spectral fitting of the donor in the low state reveals a late K-type star with $T_{\mathrm{d}\mathrm{o}\mathrm{n}\mathrm{o}\mathrm{r}}=4100\pm 150$ K. Zeeman splitting in the optical spectrum yields a candidate field strength of $B_{\mathrm{W}\mathrm{D}}=26\pm 3$ MG, meaning the system will evolve into a polar. Like other WD propellers, the system is X-ray faint ($L_X\approx 7\times {10}^{30}$ erg s${}^{-1}$), but is not detected in the radio, and a spin period is not detected in X-rays by SRG/eROSITA. The proximity of this system and evolutionary model constraints on the timeframe of the propeller phase yield a birth rate density of $\sim 8\times {10}^{-16}{\text{pc}}^{-3}{\text{yr}}^{-1}$, which agrees with that of magnetic CVs, suggesting that a substantial fraction of magnetic CVs pass through a propeller phase.