Time-resolved ultraviolet spectroscopy of the sw sex star DW UMa: Confirmation of a hidden white dwarf and the ultraviolet counterpart to phase 0.5 absorption events

Christian Knigge, University of Southampton
Sofia Araujo-Betancor, Space Telescope Science Institute
Boris T. Gänsicke, University of Warwick
Knox S. Long, Space Telescope Science Institute
Paula Szkody, University of Washington
D. W. Hoard, Spitzer Science Center
R. I. Hynes, The University of Texas at Austin
V. S. Dhillon, The University of Sheffield


We present time-resolved ultraviolet (UV) spectroscopy of the SW Sex star SW UMa in the high state. We confirm that shortward of 1500 Å, the high-state UV continuum level is lower than the white dwarf (WD) dominated low-state level. We also do not see the WD contact phases in the high-state eclipse light curves. These results confirm our earlier finding that the WD in this system is hidden from view in the high state. On the basis of this, we caution that eclipse mapping of high-inclination SW Sex stars in the high state may yield incorrect or misleading results. In the context of DW UMa, we demonstrate explicitly that distance estimates obtained by recent eclipse-mapping studies cannot be reconciled with the WD-dominated low-state spectrum. We also show that the fluxes of the UV emission lines in the high state drop near orbital phase 0.5. This is the first detection of a UV counterpart to the class-defining phase 0.5 absorption seen in the optical emission lines of SW Sex stars.