On accretion in the eclipsing polar BS Tri

Abstract

We analyse spectroscopic and photometric observations of the eclipsing polar BS Tri. The polar’s light curve shape variations can be interpreted by changing contributions of the accretion stream to the integral radiation of the system. Based on the radial velocity curves of the irradiated part of the secondary, we refine the masses of the system components, M1 = 0.60 ± 0.04M⊙, M2 ≈ 0.12M⊙, and the orbital inclination, i = 85 ± 0.5○. The polar’s spectra reveal cyclotron harmonics forming in an accretion spot with a magnetic field strength of B = 22.7 ± 0.4 MG and an average temperature of T ∼ 10 keV. In addition to the cyclotron harmonics, the BS Tri spectra contain Zeeman components of Hα line, which are probably formed in the cool halo near the accretion spot. The orientation of the magnetic dipole and the coordinates of the accretion spot are estimated by modeling the light curves of the polar. We show that for a satisfactory description of the BS Tri light curves we have to take into account the variability of the spot’s optical depth along the line of sight. Doppler maps of BS Tri show a part of the accretion stream with a trajectory close to ballistic near the Lagrange point L1, and another part of the stream moving along the magnetic field lines. The estimate of the stagnation region position found from the Doppler tomograms is consistent with the photometric estimates of the accretion spot position.

A Systematic Search for Short-period Close White Dwarf Binary Candidates Based on Gaia EDR3 Catalog and Zwicky Transient Facility Data

“…ZTF J1813+4251 is a short-period (P orb = 51.161 minutes) binary candidate near the classical-CV in the Gaia H-R diagram. ZTF J0112+5827 is a CV candidate similar to an eclipsing polar BS tri system (Kolbin et al 2022) with a period of 80.912 minutes. We obtained the ZTF photometric data of these special candidates and analyzed their positions in the Gaia H-R diagram.…”

“…14 The accreted mass moves along the magnetic lines in the direction of the white dwarf's magnetic poles. The two spikes of the light curve in one period are generated by the accretion point, which is similar to a rare eclipsing polar BS Tri system (Kolbin et al 2022). Future high-time resolution spectral observations will be needed to verify the formation of the unusual light curve of the binary system.…”

“…The two stars are close enough that the companion completely fills its Roche lobe, and the outer layer of its envelope is gradually stripped from its surface and forms an accretion disk around the white dwarf. The CVs can be further divided into several subtypes, such as: classical novae, dwarf novae, polar (AM Her-type star; Kolbin & Borisov 2020;Kolbin et al 2022), intermediate polar (DQ Her star; Norton et al 1999), and AM Canum Venaticorum system (AM CVn; Nelemans et al 2001a;Postnov & Yungelson 2014;Ramsay et al 2018;Wong & Bildsten 2021).…”

Discovery of Two Polars from a Crossmatch of ZTF and the SRG/eFEDS X-Ray Catalog

“…At this point in the orbital phase, we are seeing directly into the irradiated face of the donor, confirming the origin of the narrow emission component. This phenomenon has been known since early studies of polars (e.g., Cropper 1990) and used to constrain binary parameters of the eclipsing polar BS Tri (Kolbin et al 2022) when the traditionally used Na I 8183, 8195 doublet could not be spotted. The Na I 8183, 8195 doublet is not seen in ZTFJ0850+0443 due to the strong accretion continuum that dominates out to red optical wavelengths.…”