Accretion in Classical Novae
The outcome of the evolution of all close binary systems with an accreting white dwarfs, including different types of cataclysmic variables (CV) and the progenitors of type Ia supernovae, depends on the accretion rate and its variations in time. The two most important parameters in determining whether thermonuclear flashes occur and mass is lost from the white dwarf, are the white dwarf mass and the mass accretion rate. The X-ray luminosity is an excellent diagnostic of accretion, because the X-ray flux is mostly due to the emission of the inner regions of an accretion disk, close to the compact object, to shocks in an accretion stream funneled by the magnetic field if the white dwarf has a strong magnetic field, or to the black-body emission of hot polar caps. The highest plasma temperature that fits the hottest spectral component is a clear indication of the white dwarf mass, since the more massive the white dwarf is, the deeper its potential well. We have been able to obtain grating spectra and measure also some emission lines for a few classical novae at quiescence. X-ray grating specra are extremely useful and interesting because the line ratios allow derivng one of the three fundamental parameters determining the outcome of novae, the mass accretion rate onto the WD. For V603 Aql the grating spectrum was of very high quality (Mukai & Orio 2006), but low S/N spectra allow deriving interesting results, like for CP Pup (Orio et al. 2009). Like the old nova CP Pup, also the more recent V4743 Sgr has a massive white dwarf and is believed to be an intermediate polar. In addition to the deep exposures of these novae with XMM-Newton, we obtained a long observation of EY Cyg, a dwarf nova that is beleieved to have undergone at least one nova outburst. A different case is the one of the recurrent nova RS Oph. We observed it after it returned to quiescence, and found that the X-ray spectrum does not seem to be emitted in a disk boundary layer, however the emission may still be due to accretion through a disk with a "spread".
People: M. Orio
Collaboration: A. Bianchini (Padova Univ.), Elia Leibowitz (Tel Aviv Univ., Israel), T. Nelson
Publications: Orio et al. (2009), ApJ 690,1753