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Astron. Astrophys. 343, 455-465 (1999)

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1. Introduction

Supersoft X-Ray sources (hereafter called SSS) were discovered by Einstein, established by Rosat (Trümper et al. 1991) and are now commonly accepted to be close binary systems with a white dwarf primary. Little is known about the nature of the companion star. The accretion rate in these systems is relatively high (in the order of [FORMULA]y-1) so that the accreted hydrogen burns steadily on the surface of the white dwarf (van den Heuvel et al. 1992). Several SSS are now known, most of them in the LMC and in M31. A detailed list can be found in Greiner (1996). Only two SSS are known in our Galaxy; one of them is RX J0019.8+2156 (hereafter called RXJ0019), which was discovered by Beuermann et al. (1995).

SSS are characterized by their high X-ray luminosity from 1036 erg s-1 to 1038 erg s-1 and by extremely soft X-ray spectra with blackbody temperatures between 20 - 60 eV. Most of the optical luminosity probably originates in the bright accretion disk: in a radially extended and vertically elevated accretion disk rim a substantial fraction of the soft X-ray flux from the white dwarf is reprocessed to optical and UV-light. This model successfully accounts for features observed in the optical lightcurves of some SSS such as CAL 87, RXJ0019 and RX J0513.9-6951 (Meyer-Hofmeister et al. 1997). Most of the optical lightcurves presented in this paper have also been used by Meyer-Hofmeister et al. (1998) for their analysis. They argue that the short time variability is caused by changes in the height of the accretion disk rim. An elevated rim is therefore the premise for explaining the luminosity in optical bands. This picture of a bright accretion disk is also invoked by other authors (Popham & Di Stefano 1996, Matsumoto & Fukue 1998).

RXJ0019 is a unique object. It is the brightest and nearest among the known SSS. With its position well outside the galactic plane and its visual brightness of [FORMULA] [FORMULA] 12.5m RXJ0019 offers an outstanding opportunity to investigate a SSS very closely with indirect imaging techniques such as Doppler tomography. Here we report on such an approach to investigate RXJ0019.

In Sect. 2 we refer to our observations of RXJ0019 and the data reduction procedures used. Sect. 3 deals with the results and analysis of our data. Finally, in Sect. 4 a summary and discussion of this paper are given.

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© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999