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

Abstract

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, 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 timescales. The X-ray eclipses allow to refine the orbital ephemeris with period 0.09913398(4) d, and to constrain the binary inclination $79^{\mathrm{o}}\lesssim i\lesssim 90^{\mathrm{o}}$ and the mass ratio 0.18$<{\mathrm{M}}_2/{\mathrm{M}}_{\mathrm{WD}}<$0.40. A companion mass M${}_2=0.2-0.25{\mathrm{M}}_{\odot }$ with a radius R${}_2=0.24-0.26{\mathrm{R}}_{\odot }$ and spectral type $\sim$M4, at D$=209_{-2}^{+3}\mathrm{pc}$, is derived. A lower limit to the white dwarf mass of $\sim 0.6{\mathrm{M}}_{\odot }$ is obtained from the X-ray spectrum. An upper limit to the magnetic colatitude, $\beta \lesssim 50^{\mathrm{o}}$, and a shift in azimuth, $\psi \sim 14^{\mathrm{o}}$, 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 $\sim 0.4-1\times 10^{-10}{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, 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 thirteen members, suggesting that they are not as rare as previously believed.

Accepted for publication on MNRAS, 11 Pages, 8 Figures, 4 tables

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