We present the X-ray grating spectra of the recurrent nova RS Ophiuchi during its 2006 outburst , obtained with the XMM-Newton and Chandra observatories .
Two weeks after optical maximum , the X-ray spectrum was hard and dominated by emission lines of H-like and He-like ions .
The X-ray luminosity was 2.4 \times 10 ^ { 36 } erg cm ^ { -2 } in the 0.33-10 keV range .
The spectra indicate a collisionally dominated plasma with a broad range of temperatures .
All the lines are blue-shifted , with the velocity shift increasing with lower ionization state and longer wavelength .
Two weeks later , the spectrum was still dominated by emission lines , although the line ratios present indicate cooling .
During this observation , a soft X-ray flare occurred in which a new system of higher velocity emission lines appeared in the soft end of the spectrum .
Towards the end of the second month the emission spectrum became composite , with both absorption and emission features .
The dominant component during the third month was the supersoft continuum with the broad absorption features of a hot white dwarf atmosphere .
The absorption lines did not appear to be significantly blue-shifted as previously observed in another classical nova .
A preliminary fit of an atmospheric model available in the literature indicates a white dwarf temperature slightly exceeding 800,000 K. The X-ray luminosity reached at least 9 \times 10 ^ { 37 } erg cm ^ { -2 } in the 0.2-1 keV range , while the intrinsic nebular absorption decreased by a factor of five since the first observation .
The spectral fits indicate a massive white dwarf , with a mass of at least 1.2 M _ { \odot } .
Therefore , RS Oph may be an important type Ia supernova progenitor .
We show that the data are consistent with mass loss ending before day 54 after the outburst , and nuclear burning ending around day 69 .
A rapid decay in X-ray luminosity followed after week 10 .
The X-ray luminosity 5 , 7 and 8 months after optical maximum dropped by more than two orders of magnitude , and the spectra are dominated by soft X-ray emission lines typical of a collisionally excited and ionized plasma .
They do not appear to consistent with emission from an accretion disk .