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 exceptionalevolution

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 19.97 A (emissivity peak at T=3MK; but alsoN 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 ~1500-2000 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 thesoft emission emerges.

The soft X-ray light curve is veryvariable on short time scales all through March.This is the March 10 EPIC-pnlightcurve,at E < 0.4 keV. Variabilityless evident at 0.4-1 keV.

~35 sec period detected onlyin this portion of light curve

The lines in the soft spectrum changedramatically,ID possible onlyif 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.