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Astron. Astrophys. 353, 646-654 (2000)

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1. Introduction

Polars or AM Herculis binaries belong to the class of cataclysmic variables. They consist of a low mass main sequence star filling its critical Roche lobe and a magnetic white dwarf accreting matter from its companion. The strong magnetic field channels the accretion flow into a small area on the surface of the white dwarf. In this area the temperature rises to values of several 100 000 K and shifts the emission of reprocessed radiation into the soft X-ray regime. Therefore an efficient way to detect these systems is to search the sky for (highly) variable extreme ultraviolet or soft X-ray emitters. The great success of ROSAT, ASCA, and EUVE in detecting these systems is demonstrated by the fact, that before ROSAT only 17 Polars were known (Cropper 1990) whilst as of today this number has increased to 63. Other sources of radiation in these systems are thermal bremsstrahlung emission from the accretion column, observed at higher X-ray energies (typically 10 to 20 keV), cyclotron emission from the accretion column due to the strong magnetic field, often dominating the optical regime, and radiation from the secondary star, strongest in the infrared and sometimes only being detectable, if the system is in a state of low accretion. Detailed discussions of these systems can be found in the books by Warner (1995) and Campbell (1997). Early reviews on the basis of the new ROSAT data have been presented among others by Beuermann & Thomas (1993), Watson (1994), Beuermann & Burwitz (1995), and Schwope (1995), a more recent one by Beuermann (1997). The distribution of Polars in the solar neighborhood was investigated by Thomas & Beuermann (1997). An identification program based on a complete sample of the brightest soft X-ray sources from the RASS (Voges et al. 1999) at high galactic latitudes (Thomas et al. 1998) led to the detection of RX J1313.2-3259.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999