XMM - Newton observations of MR Vel / RX J 0925 . 7 - 4758

We report on XMM-Newton observations of the galactic supersoft X-ray source RXJ0925.7-4758. 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 kms−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 kms−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 λ 4686 and thus supports the idea that most of the He ii optical line emission arises from the accretion disk.