Phase-resolved spectroscopic observations of the magnetic cataclysmic binary EF Eridani: Revealing complex magnetic accretion during a high state

TL;DR

EF Eri, a polar, underwent a recent high-state transition after decades in quiescence. Using phase-resolved spectroscopy from SALT and SPUPNIC and applying standard and inside-out Doppler tomography along with flux-modulation mapping, the authors map the accretion structure. The analyses identify three emission regions—the irradiated secondary, the ballistic/ threading region, and a magnetically confined flow—revealing, for the first time in a polar, two distinct magnetic accretion streams feeding two poles. These results constrain the magnetic geometry and accretion dynamics in polars and highlight state-dependent shifts in line formation and flow topology, with implications for understanding magnetically controlled accretion in close binaries. The work demonstrates how high-time-resolution spectroscopy paired with tomographic techniques can uncover complex, multi-pole accretion in magnetic cataclysmic variables, informing models of field geometry and accretion physics, and it underscores the need for continued monitoring across accretion states. $P_{ ext{orb}} = 0.05626586$ d and $M_1 ext{ and } i$ estimates underpin the tomographic interpretation, situating EF Eri within the broader context of polar accretion dynamics.$

Abstract

We present high-resolution, phase-resolved spectroscopic observations of the polar EF Eri, obtained with SALT and the SAAO 1.9-m telescope during its recent emergence from a three-decade-long low state. The average spectrum shows strong emission from the Balmer lines (H$α$ and H$β$) and He~\textsc{ii} 4686 Å, along with weaker emission from the He~\textsc{i} lines and the Bowen fluorescence (C~\textsc{iii}/N~\textsc{iii}) blend at 4650 Å. The emission lines redward of 5500 Å transition to pure absorption at orbital phases $\sim$0.75--0.95, which we attribute to obscuration of the line-emitting region by the accretion stream. Trailed spectra of the emission lines reveal multicomponent structures consistent with other polars. In this first Doppler study of EF Eri, tomograms of the strongest lines (He~\textsc{ii} 4686 Å and the Balmer lines), using both the standard and inside-out projections, identify three key emission regions: the irradiated face of the secondary star, the ballistic and threading regions of the accretion stream, and the magnetically confined flow. Our Doppler maps show not only the ballistic stream but also two unambiguous magnetic accretion flows, which is consistent with the presence of multiple magnetic accretion regions.

Figures (7)

• Figure 1: Long-term ASAS-SN light curve of EF Eri spanning 11 years between 2013 November 5 and 2025 January 30 showing the extended low-state and the recent high-state. The vertical blue and green dashed lines mark the dates of the spectroscopic observations with SALT and the SAAO 1.9-m telescope, respectively.
• Figure 2: The observed trailed spectra of EF Eri based on the He ii 4686 Å line from the observations taken with the SAAO 1.9-m telescope. The colour bar to the right shows the scale with which the trailed spectra were produced.
• Figure 3: Average flux-calibrated SALT spectrum (red line) of EF Eri obtained on the nights of 2023 January 26 and 27, respectively. The spectra in green and black show the individual exposures corresponding to phase 0.79 and 0.8 and highlight the reversals in emission lines. Prominent emission lines have been labelled.
• Figure 4: Standard and inside-out Doppler tomograms as well as the observed and reconstructed trailed spectra based on the H$\alpha$ emission line from SALT observations. Top row: the standard (left) and inside-out (right) Doppler tomograms. Bottom row: observed trailed spectra (centre) and the corresponding reconstructed trailed spectra based on the standard (left) and inside-out (right) tomograms.
• Figure 5: Standard and inside-out Doppler tomograms as well as observed and reconstructed trailed spectra based on the He ii 4686 Å emission line from SALT observations. Top row: the standard (left) and inside-out (right) Doppler tomograms. Bottom row: observed trailed spectra (centre) and the corresponding reconstructed trailed spectra based on the standard (left) and inside-out (right) tomograms.