Chandra and XMM-Newton observations of RS Ophiuchi in the 2006 outburst M. Orio (INAF-Padova and U Wisconsin), T. Nelson (U Wisconsin and INAF), E. Leibowitz, J. Cassinelli (U Wisconsin), D. Prialnik, O. Yaron (U Tel Aviv), P. Mucciarelli (Univ Padova)

RS Ophiuchi outbursts: Recurrent nova. Very fast: Decay by 2 mag in 2-3 days. V~4.3 at max, V~12.5 at quiescence Outburst lasts less than a year. Similar shape of optical light curve but different time scales in each burst. Outbursts detected at all wavelengths from radio to X-ray. Interaction with giant wind makes it different from most novae (violently shocked material in X- rays and IR). Thought to be massive CO WD=> possible type Ia progenitor ???

Summary of X-ray observations: 4 RXTE observations in first 2 weeks. SWIFT XRT observations every ~2 days in February, March and April. 5 XMM-Newton observations. 1 Chandra HETG spectra in February (DDT time) 3 LETG spectra (March, April, June, DDT)+continued in the fall during decay. The EPIC-pn spectra clearly show the exceptional evolution

Two weeks after outburst: rich emission line spectrum; resonance lines of He-l like and H-like ions of Fe, S, Si, Mg and Ne. Wide range of plasma temperatures: coolest transition O VIII Lyman alpha doublet at A (emissivity peak at T=3MK; but also N VII in RGS ), hottest is Fe XXV 1s2p-1s2 resonance line (emissivity peak at T=60MK). Lines are blue shifted with velocity inversely dependent on ionization state! (Only Fe XXV is red shifted). FWHM ~ km/s, except Fe XII 3540 km/s. February: The wind

Collisional ionization or photoionization? From (f+i)/r ~1 we infer a collisionally ionized plasma.

Thermal model with different components (APEC): difficulty to explain soft RGS lines, need to assume nova wind with O and N enriched abundances, so… the colliding winds are all ejected by the nova, no red giant material? Alternative model: bow shocks in a clumpy red giant wind, like O stars (Zeta Pup model).

Comparison between XMM-Newton spectra on February 26 and March 10: the nebular emission line spectrum is still present… but it cools… and the soft emission emerges.

The soft X-ray light curve is very variable on short time scales all through March. This is the March 10 EPIC-pn lightcurve, at E < 0.4 keV. Variability less evident at keV. ~35 sec period detected only in this portion of light curve The lines in the soft spectrum change dramatically, ID possible only if v~8000 km/s.

The emerging supersoft X-ray source:

In the April RGS spectrum, pile up makes lower energy data unusable, longwards of 28 AA we have no usable data.

Model fitting: initial results  T~800,000 K at maximum, unvaried for 2 weeks  Using Rauch model for V4743 Sgr, we find evidence of high N/C (typical of CNO ashes) => some evnvelope retained  The luminosity increased in March, then remained constant until the end of April. A rapid decay followed at end- April and May Spectrum on April 20

The spectrum after May 2007: Supersoft phase ended - emission line spectrum observed with almost no continuum. Not back to accretion phase yet, and not back to minimum in X-rays.

What have we learned so far?  Still working on wind spectra: collisionally ionized plasma with multiple components and nova abundances, or more complex models necessary?  Atmospheric models indicate that the WD is very hot - around 800,000K.  35 s period discovery very important! May be spin period of WD, possibly related to collimated outflow from WD poles?  Fast cooling time, hight T(WD) imply low envelope mass => massive WD.  C/N high, data suggest C depletion => some burned material is not ejected but retained.  WD is indeed very massive and compact.  It seems to be a CO WD, as predicted in type Ia SN models.

Main aims of X-ray observations RS Oph jecta known to emit X-rays (1985, interaction with red giant wind) => it is an interesting target. Measure cooling time of the shell => indication of envelope mass and indirectly, of WD mass. Frequent outbursts with Delta(M)~8 mag can only occur on very massive white dwarfs. The WD in RS Oph is thought to be near Chandrasekhar mass, and the system is a likely type Ia SN progenitor. The central object is observable in X-rays/UV after the outburst. Gratings absolutely needed to estimate effective gravity and abundances. The gratings were used only after preliminary Swift info

The 35 sec period: summary In March and until the beginning of April, a period close to 35s was discovered in the PN lightcurve. Periods close to 35 s are also present in some, but not all, SWIFT observations around this time. P detected in sections of the lightcurve on March 10 where source becomes softer and brighter. However, instead of the WD emerging, we find an additional soft component in the wind during the interval of period detection. This period shrinks in the course of 6 weeks. It is not detected any more when mass loss ends. Shrinking of the period consistent with retracting WD atmosphere after outburst. Indications of possibly two poles, in anti-phase in different energy bands.