J. U. Ness
XMM-Newton Science Operations Centre, ESA, PO Box 78, 28691 Villanueva de la Canada, Madrid, Spain

Abstract. The 21st century X-ray observatories XMM-Newton, Chandra, and Swift gave us completely new insights into the X-ray behaviour of nova outbursts. These new-generation X-ray observatories provide particularly high spectral resolution and high density in monitoring campaigns, simultaneously in X-rays and UV/optical. The entire evolution of several nova outbursts has been observed with the Swift XRT and UVOT instruments, allowing studies of the gradual shift of the peak of the SED from UV to X-rays, time scales to the onset and duration of the X-ray brightest supersoft source (SSS) phase, and pre- and post-SSS X-ray emission. In addition, XMM-Newton and Chandra observations can efficiently be scheduled, allowing deeper studies of strategically chosen evolutionary stages. Before Swift joined in 2005, Chandra and XMM-Newton blind shots in search of SSS emission unavoidably led to some underexposed observations taken before and/or after the SSS phase. More systematic Swift studies reduced this number while increasing the number of novae. Pre- and post-SSS spectra at low and high spectral resolution were successfully modelled with collisional plasma models. Pre-SSS emission arises in shocks and post-SSS emission in radiatively cooling thin ejecta. In contrast, the grating spectra taken during the SSS phase are a lot more complex than expected and have not yet been successfully modeled. Available hot white dwarf (WD) radiation transport models give only approximate reproduction of the observations, and make some critical assumptions that are only valid in isolated WDs. More grating spectra would be important to search for systematic trends between SSS spectra and system parameters. Summary of well-established discoveries with Swift, XMM-Newton, and Chandra:

• About 50% of novae display faint X-ray emission before the start of the SSS phase
  
• The start of the SSS phase is not a smooth process. High-amplitude variations during the early SSS phase were seen that disappear close to the time when the optical plateau phase begins.
  
• The end of the SSS phase is in most cases a smooth process.
  
• The SSS grating spectra contain continuum spectra that roughly resemble a blackbody shape
  
• The SSS X-ray grating spectra of systems with known high inclination angles contain emission lines on top of the continuum
  
• The SSS X-ray spectra of systems with unknown or low inclination angles contain deep absorption lines from the interstellar medium and local, high-ionisation absorption lines that are blue shifted.