White Dwarfs in Interacting Binaries: Hydrogen Burning and Winds

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Figure 1:Artist's view of the RS Oph system at the onset of the outburst (Illustration Credit & Copyright: David A. Hardy & PPARC;  Astronomy Picture of the Day 2006 July 26).

 White dwarfs in binary systems, accreting and burning hydrogen at the very high mass accretion rate expected for type Ia SNe progenitors, have been  described in the Stars, Stellar Population and ISM section. For these studies, we use in large part X-ray observations. As described above, the two major ongoing projects focus on bright Galactic systems (usually hot post-outburst nova white dwarfs), and on X-ray binaries in external, nearby galaxies, respectively. Thus we use the two cutting-edge abilities of Chandra and XMM-Newton, high spatial resolution imaging with Chandra, and high resolution X-ray spectroscopy with both Chandra and XMM-Newton. The X-ray spectroscopy is made possible and done at the right time thanks to snapshot observations planned with Swift, and it should be possible and largely improve with future missions.

For several years we have been obtaining X-ray observations of hot and luminous post-outburst novae. In several cases the supersoft X-ray spectrum of a hot white dwarf with characteristic absorption features and edges has been obtained. Recent high resolution spectroscopy indicates the effective gravity (hence the mass assuming that the distance is known) and approximate abundances for Nova V4743 Sgr, V5116 Sgr and especially the recurrent nova RS Oph (Figure 1). The latter object has been really special also because FOR THE FIRST TIME high resolution X-ray spectra of the recurrent nova and wind were obtained (Figure 2).

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Figure 2:Spectrum of RS Oph in outburst measured with the Chandra HETG gratings on 2/26/2002, 2 weeks after the optical maximum. In red a model of multi-temperature bremsstrahlung is shown.

 In Leibowitz et al. 2009 (ApJ, submitted) we interpret it as due to high velocity material ejected in the polar direction while it is still rotating with almost the same fast revolution period of the white dwarf. The first emission lines spectra of the ejecta now need sophisticated wind modeling, possibly involving bow-shocks in a clumpy wind. We observed other classical novae in outburst after RS Oph, and also in X-ray grating spectra and light curves of V5116 Sagittarii we find variability that can be interpreted as due to different mass ejection mechanisms and condition along the polar axis. The new projects of X-rays observations of novae in outburst require phase resolved spectra, during both the orbital period and the white dwarf rotation period. This will be feasible much more often in the future with IXO, the International X-Ray Observatory. In order to study the global properties of all types of accreting and hydrogen burning white dwarfs as a stellar population, and of course also other types of close X-ray binaries (XRB), deep X-ray observations of Local Group galaxies are of great interest. We have studied almost 100 supersoft X-ray sources in M31 (Nelson & Orio, ApJ, submitted) finding clues that a significant fraction of them may reside in high mass X-ray binaries. This may explain the star formation dependence of the type Ia supernova rate without involving the single degenerate model. A new deep X-ray observation of the dwarph spheroidal Leo I has also been recently done, and is opening up new a new window to study XRB in general.

People: M. Orio, P. Mucciarelli, U. Munari, A. Siviero

Collaboration: A. Bianchini (Padova Univ.), R. DiStefano, J. Lu, F. Primini, A. Zezas (Center for Astrophysics, USA), J. Greiner (Max Planck Institut fuer Extraterrestrische Physik, Germany), T. Rauch (Tuebingen Univ., Germany), E. Leibowitz (Tel Aviv Univ., Israel), T. Nelson

Publications: Nelson et al. (2008), ApJ 673,1067; Orio et al. (2007),  ApJ 661,1105

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