MLS110213:022733+130617: a new eclipsing polar above the period gap

Abstract

This study confirms MLS110213:022733+130617 as a new eclipsing polar. We performed optical spectroscopic, polarimetric and photometric follow-up of this variable source identified by the Catalina Real Time Transient Survey. Using the mid-eclipse times, we estimated an orbital period of 3.787 h, which is above the orbital period gap of the cataclysmic variables (CVs). There are nine other known polars with longer orbital periods, and only two of them are eclipsing. We identified high- and low-brightness states and high polarization modulated with the orbital period. The spectra are typical of polars, with strong high ionization emission lines and inverted Balmer decrement. The He ii 4686 Å line is as strong as H β. We modelled the photometric and polarimetric bright-state light curves using the cyclops code. Our modelling suggests an extended emitting region on the white dwarf (WD) surface, with a mean temperature of 9 keV and B in the range 18–33, although the possibility that it could be a two-pole accretor cannot yet be ruled out. The WD mass estimated from the shock temperature is 0.67 M⊙. The derived parameters are consistent with the eclipse profile. The distance was estimated as 406 ± 54 pc using the period–luminosity–colours method. MLS110213 populates a rare sub-group of polars, near the upper limit of the period distribution, important to understand the evolution of mCVs.