2PBC J0658.0-1746: A hard X-ray eclipsing polar in the orbital period gap

The hard X-ray source 2PBC J0658.0-1746 was proposed as an eclipsing magnetic cataclysmic variable of the polar type, based on optical follow-ups. We present the first spectral and timing analysis at X-ray energies with XMM-Newton, complemented with archival X-ray, optical, infrared (IR) photometry, and spectroscopy. The X-ray emission shows bright and faint phases and total eclipses recurring every 2.38 h, consistent with optical properties. This firmly identifies 2PBC J0658.0-1746 as an eclipsing polar, the second hard X-ray selected in the orbital period gap. The X-ray orbital modulation changes from cycle-to-cycle and the X-ray flux is strongly variable over the years, implying a non-stationary mass accretion rate both on short and long time-scales. The X-ray eclipses allow to refine the orbital ephemeris with period 0.09913398(4) d, and to constrain the binary inclination 79^◦ i 90^◦ and the mass ratio 0.18< M₂/M_WD <0.40. A companion mass M₂ = 0.2 − 0.25 M with a radius R₂ = 0.24 − 0.26 R and spectral type ∼M4, at D= 209⁺₋³₂ pc, is derived. A lower limit to the white dwarf mass of ∼ 0.6 M is obtained from the X-ray spectrum. An upper limit to the magnetic colatitude, β 50^◦, and a shift in azimuth, ψ ∼ 14^◦, of the main accreting pole are also estimated. The optical/IR spectral energy distribution shows large excess in the mid-IR due to lower harmonics of cyclotron emission. A high-state mass accretion rate ∼ 0.4 − 1 × 10⁻¹⁰ M yr⁻¹, lower than that of cataclysmic variables above the gap and close to that of systems below it, is estimated. With 2PBC J0658.0-1746, the number of hard X-ray-selected polars increases to 13 members, suggesting that they are not as rare as previously believed.