  Swift X-ray observations of the ~60 day super-soft phase of the recurrent
nova RS Ophiuchi 2006 show the progress of nuclear burning on the white dwarf
in exquisite detail. First seen 26 days after the optical outburst, this phase
started with extreme variability likely due to variable absorption, although
intrinsic white dwarf variations are not excluded. About 32 days later, a
steady decline in count-rate set in. NLTE model atmosphere spectral fits during
the super-soft phase show that the effective temperature of the white dwarf
increases from ~65 eV to ~90 eV during the extreme variability phase, falling
slowly after about day 60 and more rapidly after day 80. The bolometric
luminosity is seen to be approximately constant and close to Eddington from day
45 up to day 60, the subsequent decline possibly signalling the end of
extensive nuclear burning. Before the decline, a multiply-periodic, ~35 s
modulation of the soft X-rays was present and may be the signature of a nuclear
fusion driven instability. Our measurements are consistent with a white dwarf
mass near the Chandrasekhar limit; combined with a deduced accumulation of mass
transferred from its binary companion, this leads us to suggest RS Oph is a
strong candidate for a future supernova explosion. The main uncertainty now is
whether the WD is the CO type necessary for a SN Ia. This may be confirmed by
detailed abundance analyses of spectroscopic data from the outbursts.
