THE SUPERSOFT X-RAY PHASE OF NOVA RS OPHIUCHI 2006

Abstract

Swift X-ray observations of the ~60 day super-soft phase of the recurrent\nnova RS Ophiuchi 2006 show the progress of nuclear burning on the white dwarf\nin exquisite detail. First seen 26 days after the optical outburst, this phase\nstarted with extreme variability likely due to variable absorption, although\nintrinsic white dwarf variations are not excluded. About 32 days later, a\nsteady decline in count-rate set in. NLTE model atmosphere spectral fits during\nthe super-soft phase show that the effective temperature of the white dwarf\nincreases from ~65 eV to ~90 eV during the extreme variability phase, falling\nslowly after about day 60 and more rapidly after day 80. The bolometric\nluminosity is seen to be approximately constant and close to Eddington from day\n45 up to day 60, the subsequent decline possibly signalling the end of\nextensive nuclear burning. Before the decline, a multiply-periodic, ~35 s\nmodulation of the soft X-rays was present and may be the signature of a nuclear\nfusion driven instability. Our measurements are consistent with a white dwarf\nmass near the Chandrasekhar limit; combined with a deduced accumulation of mass\ntransferred from its binary companion, this leads us to suggest RS Oph is a\nstrong candidate for a future supernova explosion. The main uncertainty now is\nwhether the WD is the CO type necessary for a SN Ia. This may be confirmed by\ndetailed abundance analyses of spectroscopic data from the outbursts.\n