Phase-Resolved Spectroscopy of the Polar V379 Vir with a Brown Dwarf

TL;DR

This paper performs phase-resolved spectroscopy of the magnetic period-bouncer V379 Vir to locate the origin of H$\alpha$ emission and to map the white dwarf's magnetic field. Through multi-epoch spectroscopy, Doppler tomography, and Zeeman analysis, it shows that H$\alpha$ emission primarily traces the accretion flow near L$_1$ rather than irradiation of the donor surface, and it derives a phase-varying magnetic field between $4.5$ and $7.5$ MG. An offset-dipole model indicates a complex magnetic topology with best-fit parameters around $i\approx 60^{\circ}$, $a\approx 0.17$, $B_0\approx 13$ MG, $\beta\approx 24^{\circ}$, and $\psi\approx 46^{\circ}$, though the model does not perfectly reproduce all features of the magnetic curve. The work underscores the importance of higher-resolution, spectropolarimetric observations to enable Zeeman-Doppler imaging and to better constrain the system's geometry and accretion physics in magnetic CVs.

Abstract

The polar V379 Vir is a well-known magnetic cataclysmic variable with a brown dwarf donor. Despite numerous studies of this system across various spectral ranges, a detailed investigation of the orbital variability of its optical spectra has not been carried out. In this work, we present an analysis of spectroscopic observations of V379 Vir obtained with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. The orbital variability of the H$α$ emission indicates that the line is most likely formed in the accretion stream near the Lagrangian point L$_1$, rather than on the donor's surface as previously assumed. The analysis of the rotational variability of the Zeeman splitting of hydrogen lines reveals a complex magnetic field topology of the white dwarf, which differs from a simple dipole configuration.

Figures (7)

• Figure 1: Example spectrum of V379 Vir (top panel) and a diagram of the Zeeman splitting of the hydrogen lines (bottom panel). The Zeeman triplets of H$\alpha$ and H$\beta$ are modeled using a set of Lorentzian profiles. Vertical lines indicate the positions of the triplet components. The inset in the top panel shows the normalized averaged spectrum in the region of the H$\beta$ line, revealing weak emission.
• Figure 2: Dynamic spectra in the regions of the H$\beta$ (left panel) and H$\alpha$ lines before (central panel) and after (right panel) subtraction of the absorption background.
• Figure 3: Left panel: Dynamic spectrum of the H$\alpha$ emission after absorption background subtraction, overlaid with the radial velocity curve of a single source. Middle panel: The same dynamic spectrum, showing the radial velocity curves of the bright slow (red sinusoid) and faint fast (blue sinusoid) components. Right panel: Orbital modulation of the integrated H$\alpha$ flux for the bright (red points) and faint (blue points) sources.
• Figure 4: Doppler tomograms of V379 Vir in the H$\alpha$ emission line, reconstructed under the assumption that the bright source forms on the irradiated surface of the donor (a), and the same tomograms rotated by an angle of $\Delta \vartheta \approx 45^\circ$ (b). The emission centroids of the bright (red cross) and faint (blue cross) components are marked. The ballistic trajectory of the accretion flow is shown by the green line, and the particle velocities along the magnetic trajectory are indicated by the red lines.
• Figure 5: Top panel: phased magnetic field curve of V379 Vir, derived from the Zeeman splitting of the H$\alpha$ and H$\beta$ lines, with an offset dipole model overplotted. Bottom panel: optical light curves of V379 Vir from ZTF and RTT-150 observations.