An in-depth study of the pre-polar candidate WX Leonis Minoris

Abstract

Optical photometry, spectroscopy, and XMM-Newton ultraviolet and X-ray observations with full phase coverage are used for an in-depth study of WX LMi, a system formerly termed a low-accretion rate polar. We find a constant low-mass accretion rate, dot{M} ∼ 1.5 × 10^-13 M_⊙ yr^-1, a peculiar accretion geometry with one spot not accessible via Roche-lobe overflow, a low temperature of the white dwarf, T_eff< 8000 K, and the secondary very likely Roche-lobe underfilling. All this lends further support to the changed view on WX LMi and related systems as detached binaries, i.e. magnetic post-common envelope binaries without significant Roche-lobe overflow in the past. The transfer rate determined here is compatible with accretion from a stellar wind. We use cyclotron spectroscopy to determine the accretion geometry and to constrain the plasma temperatures. Both cyclotron spectroscopy and X-ray plasma diagnostics reveal low plasma temperatures below 3 keV on both accretion spots. For the low-dot{m}, high-B plasma at the accretion spots in WX LMi, cyclotron cooling dominates thermal plasma radiation in the optical. Optical spectroscopy and X-ray timing reveal atmospheric, chromospheric, and coronal activity at the saturation level on the dM4.5 secondary star.