2001A&A...373..542O

We have analysed 350 pointed and serendipitous observations of 108 different classical and recurrent novae in outburst and in quiescence, contained in the ROSAT archive. One aim was to search for super-soft X-ray sources and we found only 3 of them among post-novae. Thus, the super-soft X-ray phase of novae is relatively short lived (up to 10 years) and is observed only for up to 20% of novae. Most classical and recurrent novae instead emit hard X-rays (in the ROSAT band) in the first months after the outburst, with peak X-ray luminosity of a few times 10³³erg/s. The emission, which we attribute to shocks in the nova ejecta, lasts at least ≃2 years and even much longer under special circumstances (like preexisting circumstellar material, or a prolonged wind phase). We also investigated X-ray emission due to the accretion mechanism in quiescent novae. 81 Galactic classical and recurrent novae were observed at quiescence, and only 11 were detected. Some of them are variable in X-rays on time scales of years; the X-ray spectra range from very soft to hard. The average X-ray luminosity is not larger than that of quiescent dwarf novae, even if quiescent novae are at least 10 times more luminous at optical wavelengths. There seems to be a missing boundary layer problem: a possible explanation is that boundary layer radiation in nova systems is emitted almost entirely in the extreme ultraviolet. There is no evidence of a large incidence of magnetic systems, either of enhanced X-ray luminosity in novae observed shortly before or after the outburst.