Do X-rays Make Disk Emission Lines in Quiescent Dwarf NovaequestionMark

If the standard disk instability model for outbursts in dwarf novae (DNe) iscorrect, the disk in quiescence should be cold and optically thick. Butquiescent DNe show UV and optical emission lines. Why? Is the disk opticallythin in the continuum? Does viscous dissipation near the disk photosphereproduce a corona? Or, is a corona created by Xray illumination of the disk froma hot boundary layer? Here we propose to test the Xray illumination hypothesiswith simultaneous, highly timeresolved observations of SS Cyg in quiescencewith XMMNewton and HST/STIS. The Xray flux in SS Cyg varies on shorttimescales. If Xray illumination is responsible for the emission lines, theyshould vary in response to the Xray flux. If not, one of the other mechanisms must be responsible. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]