The Supersoft X-Ray Phase of Nova Rs Ophiuchi 2006

Abstract

Swift X-ray observations of the ∼60 day supersoft phase of the recurrent nova RS Ophiuchi (RS Oph) 2006 show the progress of nuclear burning on the white dwarf (WD) in exquisite detail. First seen 26 days after the optical outburst, this phase started with extreme variability likely due to variable absorption, although intrinsic WD variations are not excluded. About 32 days later, a steady decline in count rate set in. NLTE model atmosphere spectral fits during the supersoft phase show that the effective temperature of the WD 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 signaling 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 WD mass near the Chandrasekhar limit; combined with a deduced accumulation of mass transferred from its binary companion, this leads us to suggest that 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 Type Ia supernova. This may be confirmed by detailed abundance analyses of spectroscopic data from the outbursts.

Short-period X-ray oscillations in super-soft novae and persistent super-soft sources

“…This transient signal in RS Oph was studied in more detail by Osborne et al (2011). The period was first seen on day 32.9 after outburst and last detected on day 58.8, consistent with the start of the SSS phase and the onset of the decline, respectively.…”

Swift Observations of Novae

“…In February 2006, the recurrent nova (RN) RS Oph went into outburst (references in Evans et al 2008) and a Swift monitoring campaign was begun, a mere 3.2 days after the discovery of the outburst. The right-hand panel of Figure 1 shows the fantastically detailed light-curve obtained, with the nova being followed for more than 1500 days (Osborne et al 2011). This light-curve showed that the rise-constant-fall pattern seen in the V1974 Cyg X-ray emission was an over-simplification of the emission occurring.…”

“…Left: The ROSAT light-curve of V1974 Cyg (taken from Krautter et al 1996), which had the best temporal coverage and brightest SSS detected prior to the launch of Swift. Right: The X-ray light-curve of RS Oph (Osborne et al 2011), the first detailed monitoring campaign of a nova performed by Swift. …”

“…'clumpiness' -or a change in ionization state) or by a variation in the photospheric radius. Alternatively, a change in the photospheric temperature could also have an effect (see Osborne et al 2011 for X-ray spectral fits throughout the nova outburst), with low temperatures leading to emission below the X-ray band, while higher temperatures increase the soft flux and, hence, decrease the hardness. An even higher temperature could lead to a rehardening, however, if some of the X-ray emission moves above the 2 keV limit used for the hardness ratio.…”

“…Such an oscillation could be caused by the -mechanism -a nuclear burning instability, whereby increased energy generation is followed by expansion, leading to a reduction in generation rate and hence a recontraction, which starts the cycle again. More details are given in Osborne et al (2011).…”