NLTE model atmospheres for the hottest white dwarfs: Spectral analysis of the compact component in nova V4743 Sgr

Half a year after its outburst in 2002 September, nova V4743 Sgr evolved into the brightest supersoft X-ray source in the sky with a flux maximum around 30 angstrom. We calculated grids of synthetic energy distributions based on non-local thermal equilibrium model atmospheres for the analysis of the hottest white dwarfs (WDs) and present the result of fits to Chandra and XMM-Newton grating X-ray spectra of V4743 Sgr of outstanding quality, exhibiting prominent resonance lines of C v, C VI, N VI, N VII, and O VII in absorption. The nova reached its highest effective temperature (T(eff) = 740 +/- 70 kK) around 2003 April and remained at that temperature at least until 2003 September. We conclude that the WD is massive, approximate to 1.1-1.2M(circle dot). The nuclear-burning phase lasted for 2-2.5 years after the outburst, probably the average duration for a classical nova. The photosphere of V4743 Sgr was strongly carbon deficient (approximate to 0.01 times solar) and enriched in nitrogen and oxygen (>5 times solar). Especially the very low C/N ratio indicates that the material at the WD's surface underwent thermonuclear burning. Thus, this nova retained some of the accreted material and did not eject all of it in outburst. From 2003 March to September, the nitrogen abundance is strongly decreasing; new material is probably already being accreted at this stage.