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

Kurtis Williams, Zorayda Martinez, Melissa Ornelas

TL;DR

This work reports the discovery of extremely fast, rotation-driven photometric variability in two isolated, ultra-massive, strongly magnetic white dwarfs, with periods of $P=168.941\ \mathrm{s}$ for SDSS J1557+0411 and $P=83.72\ \mathrm{s}$ for PG 1312+099. Through time-series photometry and Fourier analysis, the authors argue that surface spots on rapidly rotating WDs best explain the monoperiodic signals, while other causes such as binarity or pulsations are unlikely. The SDSS J1557+0411 result supports a crystallization-driven dynamo in an O/Ne core, with a low Rossby number $R_0 \sim 0.3$, whereas PG 1312+099 may be an unresolved double-degenerate system, indicating a complex evolutionary history. These findings constrain magnetic-field generation scenarios in massive WDs and underscore the need for larger, high-cadence surveys and follow-up spectroscopy/polarimetry to map the fast-rotator population.

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.

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

TL;DR

This work reports the discovery of extremely fast, rotation-driven photometric variability in two isolated, ultra-massive, strongly magnetic white dwarfs, with periods of for SDSS J1557+0411 and for PG 1312+099. Through time-series photometry and Fourier analysis, the authors argue that surface spots on rapidly rotating WDs best explain the monoperiodic signals, while other causes such as binarity or pulsations are unlikely. The SDSS J1557+0411 result supports a crystallization-driven dynamo in an O/Ne core, with a low Rossby number , whereas PG 1312+099 may be an unresolved double-degenerate system, indicating a complex evolutionary history. These findings constrain magnetic-field generation scenarios in massive WDs and underscore the need for larger, high-cadence surveys and follow-up spectroscopy/polarimetry to map the fast-rotator population.

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.
Paper Structure (7 sections, 2 figures)

This paper contains 7 sections, 2 figures.

Table of Contents

  1. Introduction
  2. Observations and Data Reduction
  3. Discussion
  4. Rapid rotation as the likely cause of observed variability
  5. SDSS J1557+0411: a rapidly rotating O/Ne core WD with a magnetic field generated by crystallization?
  6. The Conundrum of PG 1312+099
  7. Conclusions and Insights

Figures (2)

  • Figure 1: Periodograms for time series photometry of SDSS J1557+0411 (left panels) and PG 1312+099 (right panels). For each photometric series, an enlargement of the periodogram centered on the detected frequency is also included. A clear monoperiodic signal is detected well above the significance threshold in all cases except the $r$-band imaging of SDSS J1557+0411.
  • Figure 2: Pulse shapes for SDSS J1557+0411 (left) and WD 1312+098 (right) created by folding observed lightcurves on the primary frequency of modulation. The black curve in each panel is a sinusoid with the BG40 fit parameters from Table \ref{['tab.obsdata']}. Filled circles with error bars are points are BG40 data, open blue squares with error bars represent $g$ band data, and red filled triangles are $r$ band observations. For SDSS J1557+0411, the larger $g$ band amplitude is apparent.