A Highly Magnetic Ultramassive White Dwarf with a 23 minute Rotation Period

We present a physical characterization of Tsinghua University-Ma Huateng Telescope for Survey (TMTS) J00063798+3104160 (J0006), a rapidly rotating, ultramassive white dwarf (WD) identified in high-cadence light curves from the TMTS. A coherent 23 minute periodicity is detected in TMTS, Transiting Exoplanet Survey Satellite, and Zwicky Transient Facility photometry. A time series of low-resolution spectra with the Keck-I 10 m telescope reveals broad, shallow hydrogen absorption features indicative of an extreme magnetic field and shows no evidence for radial-velocity variations. Atmospheric modeling yields a magnetic field strength of ∼250 MG, while Gaia astrometry and photometry imply a mass of 1.06 ± 0.01 M ⊙ . A significant infrared excess is detected in the Wide-field Infrared Survey Explorer W1 band and is well fitted by a 550 K blackbody, likely arising from residual material of a merger. We interpret the 23 minute photometric modulation as the rotation period of an isolated, massive WD formed likely through the merger of a double WD binary. With one of the shortest rotation periods known among candidate merger remnants and with constraints from a deep Einstein Probe X-ray nondetection, J0006 provides a rare and important observational window into the poorly explored intermediate stages of postmerger evolution.