X-Ray and Optical Observations of Two New AM Herculis Systems Discovered by ROSAT

Abstract

Our optical identification program of bright soft high-galactic latitude sources from the ROSAT all-sky survey (|b| > 20, > 0.2 PSPC cts/s, HR1 < 0) led to the discovery of 23 new CV’s most of these are AM Her stars. We present the results of detailed X-ray and optical follow-up studies of two ROSAT discovered polars: the V~17 mag system RX J0531.5-4624 (Reinsch et al. 1994) and the V~19 mag system RX J0453.4-4213 (Burwitz et al. 1995). Optical spectroscopy reveals that the orbital period of RXJ0453 is either 102 min or 94 min. For RXJ0531 we find an orbital period of 133.5 min placing it well within the ’period gap’ of cataclysmic variables. Both objects display cyclotron emission lines from which their magnetic field strengths in the accretion region are determined as B~19 MG for RXJ0531 and B∼36 MG for RXJ0453. In the case of RXJ0453 the analysis of the phase dependent movement of the cyclotron emission line maxima contrains the accretion geometry. It gives an angle ∼85° for the sum of the orbital inclination i and the co-latitude of the accretion region β. The X-ray lightcurve in RXJ0531 is ~ 80% modulated and has two maxima per orbital cycle whereas in RXJ0453 it is dominated by flaring.