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Astron. Astrophys. 327, 1023-1038 (1997)

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1. Introduction

Cataclysmic Variables (CV's) are semi-detached close binary systems which consist of a white dwarf (primary) and a Roche-lobe-filling low mass main sequence star(secondary), which fills its critical Roche-Volume. Matter is transfered from the inner Lagrangian point via an accretion disk onto the white dwarf. A hot spot is produced where the mass stream hits the disk at the outer rim. In dwarf novae, an instability modulates either the rate of mass injection from the red star to the disk (mass transfer burst model), or the rate of mass transfer through the disk (disk instability model), producing changes in the disk's luminosity and structure (Meyer & Meyer-Hofmeister 1981, Bath & Pringle 1982). Studies of eclipsing dwarf novae allow us to measure changes in disk structure associated with outbursts and hence to test the rival theories . In the instability model during the outburst the disk is assumed to be in a hot high-viscosity state with a quasi-stationary temperature profile of about [FORMULA]. During decline from an outburst a cooling front migrates through the disk, switching the disk to a cool low-viscosity state of about 3000-4000 Kelvin. For a detailed discussion see the review article of Cannizzo ( 1993).

In this paper we discuss results from a campaign monitoring the eclipses of IP Peg during a dwarf nova outburst. We reconstructed the accretion disk in five different colours using the eclipse mapping technique. In Sect. 2, we describe the observations. Sect. 3 describes the disk reconstruction method. In Sect. 4 some predictions about cooling fronts and the corresponding reconstructions are outlined. Sect. 5 presents the eclipse maps obtained from the data. Sect. 6 discusses the uniqueness of the solutions and compares them with simulations. Appendix A presents a description of the standard eclipse mapping algorithm and problems involved. A new technique is described where the two dimensional structure of the standard algorithm is extended to a three dimensional description, thereby allowing us to model anisotropic light curve sources like the hot spot. Appendix B discusses the influence of the default image on the reconstructions.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998