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

TL;DR

This study characterizes TMTS J00063798+3104160 as an ultra-massive, magnetized white dwarf with a remarkably short rotation period of approximately 23 minutes. Through coordinated photometry (TMTS, TESS, ZTF), low-resolution spectroscopy (Keck/LRIS), Gaia astrometry, and broadband SED fitting, the authors derive a magnetic field of about $\sim$250 MG, a mass near $1.06\,M_{\odot}$, and a temperature around $2.2\times10^{4}$ K, with a substantial infrared excess modeled by a $\sim$550 K blackbody. The collective evidence supports J0006 as a merger remnant of a double WD system, containing residual merger debris and lacking X-ray emission at present, thereby offering a rare observational window into intermediate post-merger evolution and magnetic-field generation in massive WDs. The findings reinforce the link between high mass, strong magnetism, and rapid rotation in WD merger remnants while constraining possible evolutionary pathways and timescales for observable X-ray activity.

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.

Figures (7)

• Figure 1: Lomb-Scargle periodograms and folded light curves of TMTS, TESS, and ZTF $g$ and $r$ bands. In the left panels, the dashed magenta lines indicate the 3$\sigma$ or 6$\sigma$ significance levels, while the periods corresponding to the highest power are labeled. In the right panels, the phase-folded light curves are illustrated. The blue points are observed folded light-curve points. The red circles show the median flux or magnitude in phase bins to highlight the overall modulation. For display purposes, two phase cycles are plotted.
• Figure 2: Uninterrupted light curves obtained with the AZT 1.6 m telescope in the $B$, $V$, and $R$ bands. The horizontal axes represent Heliocentric Julian Date (HJD) relative to the first observation epoch, while the vertical axes display the apparent magnitudes in the $B$, $V$, and $R$ bands.
• Figure 3: Temporal series of Keck spectra of J0006. Upper-left panel: the blue side, with vertical offset. Upper-right panel: the red side. The lower-left panel corresponds to the magnetic field model to determine the magnetic field strength of the blue side, while the lower-right panel corresponds to the red side. Vertical dashed magenta lines mark the absorption lines, while the horizontal dashed lines display the 250 MG strength.
• Figure 4: Mass and cooling-age contours on the Gaia color vs. absolute-magnitude diagram. The H-rich DA model from Bedard2020 is adopted. The red dot marks the location of J0006. The contours from upper right to lower left are WD masses from 0.21 M$_{\odot}$ to 1.29 M$_{\odot}$. The contours from middle left to lower right are cooling ages from 1 Gyr to 6 Gyr.
• Figure 5: SED fitting for J0006. The green dots represent data from SDSS in the $u$, $g$, $r$, $i$, and $z$ bands. Yellow crosses indicate data from Pan-Starrs in the $g$, $r$, $i$, $z$, and $y$ bands. The best-fit model spectrum corresponding to 25,000 K is overplotted.