Introduction

Classical nova explosions are the third most violent explosions that can occur in a galaxy, exceeded only by a supernova explosion and a y-ray burst. With total liberated energies of more than 1045 erg s^1 novae are less energetic than supernovae but nova outbursts are far more frequent in a galaxy than supernovae. According to the now commonly accepted standard model, nova explosions happen in cataclysmic binary systems that are close binaries with one member, a white dwarf, and the other member, a main sequence or slightly evolved star that fills its Roche lobe. Mass transfer from the secondary to the white dwarf through the inner Lagrangian point leads to the formation of an accretion disk. Ultimately, because of viscous processes, the material ends up on the surface of the white dwarf. The accreted layer of hydrogen-rich material on top of the white dwarf will grow in thickness and temperature and density will increase. If a certain critical pressure is reached at the bottom of the accreted envelope (which must be, at least partially, degenerate), explosive thermonuclear burning of hydrogen via the CNO cycle will start. The evolution of this "thermonuclear runaway" (TNR) depends upon the mass and luminosity of the white dwarf, the mass accretion rate, and the chemical composition of the accreted layer (cf. e.g., [33]). The temperature at the hydrogen burning zone will grow to values exceeding 108 K. Luminosities are of the order of the Eddington luminosity LEdd, peak luminosities in the early phases after the onset of the TNR can even be in excess of LEdd by factors up to ten. Convection turns on and transports the energy generated to the surface of the accreted envelope. Radioactive decay of ^+-active nuclei eventually provides the energy to eject the nova shell. Ejection velocities are between several hundred and several thousand kilometer per second. According to the TNR model the entire character of the outburst, light curve, ejection velocities, and timescale of the outburst evolution depend on the amount of CNO nuclei present in the accreted envelope (Starrfield & Sparks [34]). One of the predictions of the TNR model was that novae should have strong overabundances of CNO nuclei, which was later verified to a full extent. The CO nuclei are supposed to be mixed from the white dwarf into the accreted envelope. Abundance studies of novae revealed the existence of a new class of white dwarfs; about 25% of all novae show strong overabundances of O, Ne, and Mg that come from a white dwarf with an ONeMg core. Novae are empirically classified by the decay time t3, the time it takes for the visual brightness to decline from maximum brightness by three magnitudes, which is a measure of the violence of the outburst. Fast novae have t3 < 100d, slow novae t3 > 100d. According to the number of outbursts observed, one distinguishes between classical and recurrent novae. For recurrent novae two or more outburst were recorded, while for classical novae only one outburst was observed. In the following, we shall concentrate on classical novae that are much more frequent than recurrent novae. However, it should be mentioned that on longer timescales all novae are believed to be recurrent.

X-ray observations have turned out to be a very powerful tool to study the outburst of novae, since the X-ray regime is best suited to study the hot phases in a nova outburst. X-ray observations have provided many fundamental and in part unexpected results. However, so far the picture that emerged from X-ray observations of novae is far less systematic than the one from other spectral regimes, since unlike in the optical, infrared, or ultraviolet regime, only few objects were observed in X-rays.

The progress in our knowledge of X-rays from novae is closely related to the advances in sensitivity and spectral resolution of subsequent gererations of X-ray satellites. There are three milestones in the observational X-ray history of novae: the first nova in outburst was detected with EXOSAT, ROSAT and its much superior sensitivity established many basic X-ray properties, and the next big step forward came with XMM and Chandra particularly through their grating spectrometers with a resolution of up to a thousand (R/AR). Reviews of X-ray observations of novae in outburst were given by Krautter [13,14], Ogelman & Orio [22], and Orio [29].

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