The Rapid Rotation of the Strongly Magnetic Ultramassive White Dwarf EGGR 156

TL;DR

This paper reports the discovery of rapid rotation in the strongly magnetic, ultramassive white dwarf EGGR 156, suggesting it is a merger remnant, and discusses how period changes can inform white dwarf physics.

Contribution

It presents the first measurement of a 22.4-minute rotation period in EGGR 156, linking rapid rotation to white dwarf mergers and analyzing magnetic dipole radiation effects.

Findings

EGGR 156 exhibits a 22.4-minute rotation period.

Magnetic dipole radiation causes period changes in massive white dwarfs.

Some white dwarfs could serve as precise clocks due to negligible period change.

Abstract

The distribution of white dwarf rotation periods provides a means for constraining angular momentum evolution during the late stages of stellar evolution, as well as insight into the physics and remnants of double degenerate mergers. Although the rotational distribution of low mass white dwarfs is relatively well constrained via asteroseismology, that of high mass white dwarfs, which can arise from either intermediate mass stellar evolution or white dwarf mergers, is not. Photometric variability in white dwarfs due to rotation of a spotted star is rapidly increasing the sample size of high mass white dwarfs with measured rotation periods. We present the discovery of 22.4 minute photometric variability in the lightcurve of EGGR 156, a strongly magnetic, ultramassive white dwarf. We interpret this variability as rapid rotation, and our data suggest that EGGR 156 is the remnant of a double degenerate merger. Finally, we calculate the rate of period change in rapidly rotating, massive, magnetic WDs due to magnetic dipole radiation. In many cases, including EGGR 156, the period change is not currently detectable over reasonable timescales, indicating that these WDs could be very precise clocks. For the most highly magnetic, rapidly rotating massive WDs, such as ZTF J1901+1450 and RE J0317$-$853, the period change should be detectable and may help constrain the structure and evolution of these exotic white dwarfs.