Sources of XRays

There are several mechanisms for X-ray emission from a nova in outburst. The most obvious one is thermal radiation from the hot white dwarf. First, one expects thermal radiation during the very early phases of the outburst. After the energy created by the nuclear reactions has reached the surface of the white dwarf, the effective surface temperature increases rapidly. With a luminosity of the order of LEdd and a white dwarf radius one expects temperatures of several hundred thousand K, depending on the white dwarf mass (and, thereby, radius). The nova emits strong soft X-rays with the spectral energy distribution (SED) of a hot stellar atmosphere. After the ejection of the nova shell the temperature of the expanding pseudophotosphere decreases rapidly with increasing radius and the X-ray flux drops, since the expanding envelope becomes opaque to X-rays. Since the duration of the soft X-ray emission during this so called "fireball phase" is very short, of the order of a few hours only, no X-rays have ever been observed during this phase. Also in future it will be very difficult to observe a nova in X-rays during the fireball phase.

A second phase of X-ray emission from the hot white dwarf is found in later phases of the outburst. As the TNR model predicts, only part of the ejected shell reaches velocities higher than escape velocity. The remaining material quickly returns to quasistatic equilibrium and forms an envelope around the cataclysmic binary system with the initial dimensions of a giant star. The matter in this quasistatic envelope supplies the fuel for the ongoing hydrogen burning (via the CNO cycle), which takes place at constant luminosity on top of the white dwarf. As evolution proceeds during this "constant bolometric luminosity phase," the residual hydrogen-rich envelope matter will be consumed by hydrogen burning, radiation driven mass loss, and dynamical friction. The radius of the stellar photosphere shrinks in time at a constant luminosity of ~LEdd so that the temperature increases up to several hundred thousand Kelvin, depending on the white dwarf mass. The peak of the nova's observed SED shifts from visual to ultraviolet and finally to the X-ray energy band. The nova becomes again a strong X-ray emitter with a soft SED of a hot stellar atmosphere. The duration of the phase of constant bolometric luminosity should be an inverse function of the white dwarf mass [12,35].

The second general mechanism is X-ray emission from the circumstellar material surrounding the nova system where the expanding nova shell and/or a nova wind interact with preexisting material or with each other. Here one expects strong shocks giving rise to a thermal bremsstrahlung SED with temperatures up to several kiloelectronvolt. Balman, Krautter and Ogelman [2] have summarized the different ways by which the X-rays can be produced by shocks.

Two further possible mechanisms for X-ray radiation should be shortly mentioned: After reestablishment of accretion one expects X-ray emission typical for cataclysmic variables in quiescence with a thermal bremsstrahlung spectrum. Some evidence for such an X-ray emission was found by Hernanz and Sala [8] for V2487 Oph (1998). A mechanism of a totally different nature was suggested by Livio et al. [15], namely that downgradation of gamma rays produced by radioactive decay of Na22 could give rise to X-rays. However, no evidence for this mechanism has been found so far.

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