UU Aqr from high to low state

Abstract

In this paper we present Physical Parameter Eclipse Mapping (PPEM) of UBVRI eclipse light curves of UU Aqr from high to low states. We used a simple, pure hydrogen LTE model to derive the temperature and surface density distribution in the accretion disc. The reconstructed effective temperatures in the disc range between 9000K and 15000K in the inner part of the disc and below 7000K in the outer parts. In the higher states it shows a more or less prominent bright spot with T_eff between about 7000K and 8000K. The inner part of the disc (R < 0.3R_L1) is optically thick at all times, while the outer parts of the disc up to the disc edge (0.51+-0.04R_L1 in the high state and 0.40+-0.03R_L1 in the low state) deviate from a simple black body spectrum indicating that either the outer disc is optically thin or it shows a temperature inversion in the vertical direction. While during high state the disc is variable, it appears rather stable in low state. The variation during high state affects the size of the optically thick part of the disc, the white dwarf or boundary layer temperature and the uneclipsed component (originating in a disc chromosphere and/or cool disc wind), while the actual size of the disc remains constant. The difference between high and low state is expressed as a change in disc size that also affects the size of the optically thick part of the disc and the presence of the bright spot. Using the PPEM method we retrieve a distance for UU Aqr of 207+-10pc, compatible with previous estimates.