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

TL;DR

The study presents phase-resolved optical spectroscopy and multi-band photometry of ZTF 1851+1714, confirming a WD spin period of $12.37$ min and revealing hydrogen-rich emission lines atop a variable continuum that reddens when brighter. Phase-resolved Doppler shifts in Hα and Hβ, with differing amplitudes, point to emission from distinct regions in a magnetically controlled accretion flow, consistent with an accretion curtain in an intermediate polar. FFT analyses suggest possible orbital sidebands near $3.88$ hr$^{-1}$ and $7.76$ hr$^{-1}$, implying a potential orbital period around $1.00$ hr, though this remains uncertain due to aliasing. The results support classifying ZTF 1851+1714 as a magnetic CV in the IP regime, while highlighting the need for longer, multiwavelength observations (including X-ray and polarimetry) to robustly determine the orbital period and magnetic geometry.

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 H$α$ and H$β$ lines exhibit different radial velocity amplitudes, suggesting that they come from different emission regions. We also identify a candidate orbital period of 1.00 hr, 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.

Figures (8)

• Figure 1: Stacked spectrum using all exposures in all grisms. The blue section represents the grism 18 observation, the black spectrum the weighted mean of the 40 grism 19 spectra, and the red section represents the grism 20 spectrum. The positions of emission lines from Hi, Hei, and Heii are indicated with vertical lines at the top. Some of the hydrogen lines in the grism 20 spectrum are only tentatively detected. We also mark the position of NaD absorption, which is presumably interstellar in nature. The spectrum has been normalised to 1 at 6000 Å.
• Figure 2: Phase-resolved spectroscopy covering the spectral region from 4400 Å to 6770 Å using grism 19. We show the stacked spectra for each of the given groups covering the full phase of the 12.37 min variability period.
• Figure 3: Velocities derived from Doppler shifts measured from the H$\alpha$ and H$\beta$ lines covering the full phase of the 12.37 min variability period (see Fig. \ref{['fig:spectra_phased']}). Sine curves have been fitted to the H$\alpha$ and H$\beta$ data points to highlight their periodic nature. For comparison, we also show velocities measured from sky-lines in the same spectra.
• Figure 4: Calculated integrated flux for each of the 40 spectra. Each spectrum has been numerically integrated in their wavelength range. The time assigned to each spectrum is the start time of the exposure. These timestamps were shifted such that the first measurement starts at 0.
• Figure 5: Slopes of the continua for the different spectra, obtained by fitting a straight line to each spectrum. The values are scaled in terms of the continuum with the highest slope, such that this value equals one.