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The end of the super-soft X-ray phase of the recurrent nova RS Ophiuchi

ATel #838; J Osborne, K Page, A Beardmore, M Goad (Leicester), M Bode (Liverpool John Moores), T O'Brien (Manchester), G Schwarz (West Chester), S Starrfield, J-U Ness (Arizona State), J Krautter (Heidelberg), J Drake (SAO), A Evans (Keele), S P S Eyres (Central Lancashire)
on 14 Jun 2006; 17:03 UT
Credential Certification: Julian P Osborne (julo@star.le.ac.uk)

Subjects: X-ray, Nova

Swift X-ray telescope observations of the recurrent nova RS Oph showed a linear decline in soft X-ray flux from a plateau level between Apr 12.1 and Apr 28.7 (ATEL #801). Since then Swift observations have continued, initially at nominal intervals of 2 days, and later, 4 days.

The bright super-soft X-ray phase described in ATEL #770 (also CBET #498) appears to be over. The 0.3-10 keV count rate started to flatten out from its linear decline between the observations on May 10.6 and May 14.5. From May 31.0 to the most recent observation on Jun 11.0 the flux in this band has been almost constant, at around 0.3 c/s. In addition, the (1-10)/(0.3-1) keV hardness ratio entered a plateau between the observations on May 21.3 and May 24.3, at a level of ~1, close to that seen before the start of this phase on Mar 13.8 (ATEL #764), and quite distinct from the ratio of ~0.01 seen during the bright super-soft phase.

The flux above 1 keV has declined almost monotonically since Swift observations began, likely due to the cooling shocked red giant wind.

The duration of the bright super-soft X-ray phase of RS Oph for this 2006 nova is of order 60 days. This is less than all the measured durations listed by Greiner et al (A&A 405, 703, 2003), although comparable to the upper limit for N LMC 2004 (for which no super-soft phase was seen). The short duration follows the general trend of decreasing duration with increasing white dwarf mass, as the white dwarf in RS Oph is believed to be close to the Chandrasekhar mass (e.g. Shore et al ApJ 456, 717, 1996). Hachisu & Kato (astro-ph/0602563) make detailed predictions for the start and end dates of the super-soft phase in RS Oph based on various white dwarf masses and compositions, however the all the models listed predict a super-soft phase start after that observed, and models with early start dates have super-soft phases much shorter than observed.

We thank the Swift PI, Neil Gehrels, the Swift science team, and the Swift mission operations team for their support of this observing campaign.