Unraveling the puzzle of the eclipsing Polar SDSS J015543.40+002807.2 with XMM and optical photometry/spectropolarimetry

The cataclysmic variable SDSS J015543.40+002807.2 is confirmed to be a magnetic system of the AM Herculis type. With an orbital period of 87.13 minutes, it is also the shortest period eclipsing Polar known. Monitoring with XMM-Newton finds a high-state light curve dominated by a single X-ray-emitting accretion pole located slightly prograde of the secondary star. The hard X-ray spectrum is typical of radial shocks on magnetic white dwarfs (kT∼10 keV), and there is evidence for a soft X-ray component consistent with reprocessing from the stellar surface. The optical circular polarization is weak (v ≲3%) when the accretion rate is high (mv ∼ 15.5), as a result of optically thick cyclotron emission and the apparent competition between two accreting poles. However, in low states (mv ∼ 18), the polarization increases smoothly to the blue, reaching 20% at 4200 Å, and the flux spectrum displays a rich set of thermally broadened cyclotron harmonics that indicate a polar field of 29 MG. The phase interval preceding the 6.5 minute eclipse depicts the development of P Cygni components followed by complete absorption reversals in the emission lines. This phenomenon is not unexpected for a strongly accreting magnetic system viewed through the cool base of the funnel, and high-quality spectroscopy through this interval will likely lead to new insights into the dynamics of magnetic coupling and gas flow onto the white dwarf.