Phase-resolved optical spectroscopy of the rapidly varying white dwarf ZTF 1851+1714

¹ Department of Materials and Production, Aalborg University, Fibigerstræde 16, 9220 Aalborg, Denmark
² Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
³ Cosmic DAWN Center
⁴ INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello16, I-80131 Napoli, Italy
⁵ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁶ Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, local 5, E-38711 San Antonio, Breña Baja, Santa Cruz de Tenerife, Spain

^⋆ Corresponding author: casper.c.pedersen@gmail.com

Received: 11 April 2025
Accepted: 29 September 2025

Abstract

We report on phase-resolved optical spectroscopy and photometry in the R and B bands of the white dwarf candidate ZTF 185139.81+171430.3. The source has been reported to be variable with a large amplitude of close to 1 magnitude, in the R band, and a short period of 12.37 min. We confirm this period and interpret it as the spin period of the white dwarf. The optical spectrum shows emission lines from hydrogen and helium superposed on a featureless continuum. The continuum changes shape throughout a cycle, such that it is redder when the source is bright. There is tentative evidence for Doppler shifts in the emission lines during the spin cycle with an amplitude of a few tens of km s⁻¹. Notably, the Hα and Hβ lines exhibit different radial velocity amplitudes, suggesting that they come from different emission regions. We also identify a candidate orbital period of 1.00 h, based on potential orbital sidebands. These features – Doppler shifts modulated at the spin frequency, brightness variations, and continuum shape changes – are consistent with the accretion curtain model, in which material is funneled from a truncated inner disc along magnetic field lines onto the magnetic poles of the white dwarf.