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Astron. Astrophys. 356, 490-500 (2000)

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1. Introduction

Polars, as AM Hers are commonly named due to their highly polarized emission, consist of a late type main-sequence star (red dwarf, secondary star) and a highly magnetized white dwarf (WD, primary star). The red dwarf, filling its Roche volume, injects matter through the [FORMULA] point into the Roche volume of the WD. Unlike in non-magnetic systems, this material does not form an accretion disc, but couples onto the magnetic field once the magnetic pressure exceeds the ram pressure. From this stagnation region (SR) on, the accretion stream follows a magnetic field line until it impacts onto the white dwarf surface. For a review of polars, see Warner (1995).

In systems with an inclination [FORMULA], the secondary star gradually eclipses the accretion stream during the inferior conjunction. Using tomographical methods, it is - in principle - possible to reconstruct the surface brightness distribution on the accretion stream from time resolved observations. This method has been successfully applied to accretion discs in non-magnetic CVs ("eclipse mapping", Horne 1985). We present tests and a first application of a new eclipse mapping code, which allows the reconstruction of the intensity distribution on the accretion stream in magnetic CVs.

Similar attempts to map accretion streams in polars have been investigated by Hakala (1995) and Vrielmann & Schwope (1999) for HU Aquarii. An improved version of Hakala's 1995 method has been presented by Harrop-Allin et al. (1999b) with application to real data for the system HU Aquarii (Harrop-Allin et al. 1999a). A drawback of all these approaches is that they only consider the eclipse of the accretion stream by the secondary star. In reality, the geometry may be more complicated: the far side of the magnetically coupled stream may eclipse stream elements close to the WD as well as the hot accretion spot on the WD itself. The latter effect is commonly observed as a dip in the soft X-ray light curves prior to the eclipse (e.g. Sirk & Howell 1998). The stream-stream eclipse may be detected in data which are dominated by emission from the accretion stream, e.g. in the light curves of high-excitation emission lines where the secondary contributes only little to the line flux.

Here, we describe a new accretion stream eclipse mapping method, using a 3d code which can handle the full complexity of the geometry together with an evolution strategy as fit algorithm. We present extensive tests of the method and map as a first application to real data the accretion stream in UZ For emission of C IV [FORMULA] 1550.

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© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000
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