A Highly Magnetic Ultra Massive White Dwarf with a 23-minute Rotation Period

TL;DR

This paper reports the discovery and detailed analysis of a rapidly rotating, ultra-massive, highly magnetic white dwarf with a 23-minute rotation period, likely formed through a merger, providing insights into post-merger evolution.

Contribution

It presents the first detailed characterization of a highly magnetic, ultra-massive white dwarf with an extremely short rotation period, linking it to merger origins.

Findings

Magnetic field strength of approximately 250 MG.

Mass of the white dwarf is 1.06 solar masses.

Infrared excess suggests residual merger material.

Abstract

We present a physical characterization of TMTS J00063798+3104160 (J0006), a rapidly rotating,ultra-massive white dwarf (WD) identified in high-cadence light curves from the Tsinghua University-Ma Huateng Telescope for Survey (TMTS). A coherent 23-minute periodicity is detected in TMTS, TESS, and ZTF 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 $\sim$ 250 MG, while Gaia astrometry and photometry imply a mass of 1.06 $\pm$ 0.01 M$_{\odot}$. A significant infrared excess is detected in the WISE 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 post-merger evolution.