An Isolated White Dwarf with a 70 second Spin Period

TL;DR

This paper reports the discovery of the fastest spinning isolated white dwarf with a 70-second period, suggesting a merger origin and opening new avenues for studying white dwarf companions and pulsation phenomena.

Contribution

It presents the first detailed observation and analysis of a 70-second spin period in an isolated white dwarf, indicating a merger remnant and expanding understanding of white dwarf rotation.

Findings

Discovered the fastest spinning isolated white dwarf to date.

Identified a magnetic field strength of 15 MG and mixed atmospheric composition.

Suggests some ZZ Ceti variables may be fast-spinning white dwarfs.

Abstract

We report the discovery of an isolated white dwarf with a spin period of 70 s. We obtained high speed photometry of three ultramassive white dwarfs within 100 pc, and discovered significant variability in one. SDSS J221141.80+113604.4 is a $1.27~M_{\odot}$ (assuming a CO core) magnetic white dwarf that shows 2.9\% brightness variations in the BG40 filter with a $70.32 \pm 0.04$ s period, becoming the fastest spinning isolated white dwarf currently known. A detailed model atmosphere analysis shows that it has a mixed hydrogen and helium atmosphere with a dipole field strength of $B_d = 15$ MG. Given its large mass, fast rotation, strong magnetic field, unusual atmospheric composition, and relatively large tangential velocity for its cooling age, J2211+1136 displays all of the signatures of a double white dwarf merger remnant. Long term monitoring of the spin evolution of J2211+1136 and other fast spinning isolated white dwarfs opens a new discovery space for substellar and planetary mass companions around white dwarfs. In addition, the discovery of such fast rotators outside of the ZZ Ceti instability strip suggests that some should also exist within the strip. Hence, some of the mono-periodic variables found within the instability strip may be fast spinning white dwarfs impersonating ZZ Ceti pulsators.