84 Second and 169 Second Rotation of Two Isolated, Ultramassive, Strongly Magnetic White Dwarfs

TL;DR

This paper reports the discovery of extremely rapid rotation periods in two magnetic white dwarfs, challenging existing models of white dwarf rotation and suggesting new insights into their magnetic and evolutionary properties.

Contribution

The study presents the first detection of sub-minute rotation periods in two ultramassive, magnetic white dwarfs, providing evidence for rapid rotation and potential binary nature.

Findings

SDSS J1557+0411 rotates with a period of 168.94 seconds.

PG 1312+099 rotates with a period of 83.72 seconds.

One white dwarf may be an unresolved double degenerate system.

Abstract

Photometric variability in massive, magnetic white dwarfs (WDs) on the timescales of less than a few hours is oft interpreted as being due to magnetic spots on the surface of a rotating star. Increasingly, numbers of these short period variables are being discovered with the continued growth of time-domain astronomy, testing theories of magnetic white dwarf formation and angular momentum evolution. We present the detection of extremely rapid rotation in the WDs SDSS J1557+0411 and PG 1312+099, with periods of 168.94 s and 83.72 s, respectively. We consider other possible causes of the monoperiodic photometric variability, including binarity (eclipses and ellipsoidal variations) and asteroseismic pulsations. Though these cannot be ruled out with the existing data, these alternative explanations seem unlikely. SDSS J1557+0411 was predicted to be a rapidly rotating oxygen-neon core WD by Camisassa et al. (2022), a prediction borne out at least in part by our observations. PG 1312+099 was previously observed via polarimetry to be rotating with a period of ~5.4 h; we propose that this object may be an unresolved double degenerate system.