Luminous Supersoft X-ray Quasars

We will make short observations of 18 sources in a newly identified class ofobjects we call supersoft Xray quasars (SXQs). These are highluminosityquasars (Lx virgul4x10^44 10^46 erg/s) with extremely soft (i.e., steep) spectrathat were discovered during the ROSAT AllSky Survey. The proposed XMMobservations will more than double the number of such objects for which spectraare available. In addition, we will observe for 40 ks one SXQ with a powerlawspectral index of virgul7. Our objectives are (i) to characterize the spectra of SXQsover the full range of 0.210 keV, and (ii) to measure a multicomponent thermaldisk spectrum. The latter will provide a direct handle on the black hole massand spin. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]