Detection of a weak magnetic field in the Balmer emission line white dwarf WDJ1653-1001

TL;DR

This paper reports the first detection of a weak magnetic field in a DAe white dwarf, WDJ1653-1001, revealing its magnetic nature and suggesting a unified mechanism behind similar white dwarf classes.

Contribution

It presents the discovery and characterization of magnetism in a DAe white dwarf, linking it to the DAHe class and proposing a common physical origin.

Findings

Detected a weak, variable magnetic field in WDJ1653-1001.

Observed an 80-hour periodicity in photometry and spectral emission.

Found anti-phase variation between photometric flux and Balmer emission.

Abstract

The small DAHe and DAe spectral classes comprise isolated, hydrogen-dominated atmosphere white dwarfs that exhibit variable photometric flux and Balmer line emission. These mysterious systems offer unique insight into the complex interplay between magnetic fields, stellar rotation and atmospheric activity in single white dwarfs. DAHe stars have detectable magnetic fields through Zeeman-split spectral lines, whereas DAe stars lack such splitting. We report the first discovery and characterisation of magnetism in the DAe white dwarf WDJ165335.21-100116.33 with new time-resolved spectropolarimetry from FORS2. We detect a weak but variable longitudinal magnetic field with values $\langle B_z \rangle > -9.2 \pm 2.4$ kG and $\langle B_z \rangle < -2.2 \pm 1.0$ kG. Independent ZTF and ATLAS photometry reveal a consistent period of P = 80.3070 $\pm$ 0.0007 h. Time-resolved optical spectroscopy obtained with six ground-based instruments demonstrates strong modulation in the strength of the H$\alpha$ and H$\beta$ Balmer line emission with P = 80.2922 $\pm$ 0.0108 h. The photometric flux and Balmer emission strength vary in antiphase, with the strongest magnetic detections coinciding with phases of low photometric flux and strong line emission. These characteristics support the theory that a magnetically active, temperature-inverted spot/region is producing an optically thin chromospheric emission region. Comparison with other DAe and DAHe white dwarfs reveals all systems have a strikingly similar antiphase phenomenology, reinforcing the theory that they are subject to a unified physical mechanism. With the detection of a weak magnetic field, we reclassify WDJ165335.21-100116.33 as a low-field DAHe white dwarf.