XMM-Newton observations of MR Vel/RX J0925.7-4758

We report on XMM-Newton observations of the galactic supersoft X-ray source RX J0925.7-4758 . The RGS spectrum exhibits a wealth of spectral features from iron and oxygen. XMM-Newton data confirm the finding of previous Chandra HETGS/MEG observations that NLTE models of hot white dwarf atmospheres fail to represent the complex spectrum. There are clear evidences for P Cygni profiles with wind velocities of up to 2000 km s^-1. Small flux variations with time scales larger than 1000 s are present. The strongest power is at ~ 0.21 d, a period close to that seen in V band optical light curves. A detailed analysis of the associated changes in the RGS and EPIC pn spectra hint at a mostly grey mechanism suggesting a variation of the visibility of the white dwarf due to occulting material in the accretion disk. Finally, we detect radial velocity changes of 173 +/- 47 km s^-1 between two RGS observations obtained half an orbital cycle apart. The amplitude of the RGS velocity shift is consistent with that of the optical He II lambda 4686 and thus supports the idea that most of the He II optical line emission arises from the accretion disk.