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

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$α$ and H$β$ 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.

Figures (17)

• Figure 1: Power spectra computed from ZTF (a and b) and ATLAS (c and d) broad-band photometric data of WD J1653$-$1001. The strongest signal common to all power spectra corresponds to a period of $P \simeq 80.30 - 80.31$ h, which is detected above a FAP of $5\sigma$ (solid horizontal lines). The DR23 ZTF $r$-band (red), $g$-band (green) and combined $g$- and $r$-band (black) data are shown in panels a and b, where b shows the power spectra zoomed in on the periodic signal at $P \simeq 80.30$ h. The ATLAS $c$-band (blue), $o$-band (orange) and combined $c$- and $o$-band (black) data are shown in panels c and d, with d similarly zoomed in. The legends apply to both panels in each dataset.
• Figure 2: ZTF and ATLAS light curves of WD J1653$-$1001. (a) shows the ZTF $g$-band (green), $r$-band (red) and combined $g$- and $r$-band (black) light curves. (b) shows the ATLAS $c$-band (cyan), $o$-band (orange) and combined $c$- and $o$-band (black) light curves. (c) upper panel shows the combined $g$-, $r$-, $o$- and $c$-band (grey) light curve, fitted with a sinusoid (black solid line). ${\rm Phase} = 0$ corresponds to the photometric maximum at the ephemeris in Eq \ref{['ZTF_ATLAS_photometric_ephemeris']}. All individual band light curves are binned into 150 data points and all combined light curves are binned into 30 data points. Each light curve is phase-folded onto a period of $80.3070$ h and data are repeated over two phases for illustrative purposes. Error bars represent the 1$\sigma$ scatter in each bin. (c) lower panel shows the spectroscopic phase coverage achieved by observations from Binospec (yellow), FORS2 2024 (red), MagE (green), INT (blue), Kast (purple), MIKE (orange) and FORS2 2022 (pink).
• Figure 3: Power spectra computed from Calar Alto and ASAS-SN photometric data of WD J1653$-$1001. A consistent signal of $P \simeq 80.3$ h (grey vertical line) is detected above a FAP of $5\sigma$ (solid horizontal lines) in the $B$ (blue), $V$ (green), $R$ (red) and combined (black) filter data from Calar Alto, and also in the ASAS-SN (purple) data.
• Figure 4: Two spectra of WD J1653$-$1001 taken with the FORS2 spectrograph on 2022-05-15 and 2022-06-01 around the H$\beta$ Balmer line region. The observation UT dates are shown on the right of the plot. Spectra are convolved with a Gaussian with a FWHM of 1 Å and offset vertically for clarity.
• Figure 5: Six spectra of WD J1653$-$1001 taken with the FORS2 spectrograph between 2024 May-July around the H$\alpha$ Balmer line region. The observation UT dates are shown on the right of the plot. Spectra are convolved with a Gaussian with a FWHM of 1 Å and offset vertically for clarity.