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Models of nova outbursts suggest that an X-ray flash should occur just after hydrogen ignition. However, this X-ray flash has never been observationally confirmed. We present four theoretical light curves of the X-ray flash for two very massive white dwarfs (WDs) of 1.380 and 1.385 and for two recurrence periods of 0.5 and 1 yr. The duration of the X-ray flash is shorter for a more massive WD and for a longer recurrence period. The shortest duration of 14 hr (0.6 days) among the four cases is obtained for the WD with a 1 yr recurrence period. In general, a nova explosion is relatively weak for a very short recurrence period, which results in a rather slow evolution toward the optical peak. This slow timescale and the predictability of very short recurrence period novae give us a chance to observe X-ray flashes of recurrent novae. In this context, we report the first attempt, using the Swift observatory, to detect an X-ray flash of the recurrent nova M31N 2008-12a (0.5 or 1 yr recurrence period), which resulted in the nondetection of X-ray emission during the period of 8 days before the optical detection. We discuss the impact of these observations on nova outburst theory. The X-ray flash is one of the last frontiers of nova studies, and its detection is essential for understanding the pre-optical-maximum phase. We encourage further observations. [ABSTRACT FROM AUTHOR]

Classical novae are stellar explosions in cataclysmic binary systems, consisting of a compact white dwarf star (CO or ONe-rich) and a low-mass, main-sequence companion (typically, a K or M dwarf of solar composition). The system is close enough (orbital periods ranging between 1 and 15 h), hence allowing mass transfer episodes driven by overflows of the companion star. This matter flow forms an accretion disk that surrounds the white dwarf, and ultimately accumulates on its surface (at a rate M = 10[?]9-10[?]10 M[?] yr[?]1), building up an envelope under semi-degenerate conditions until a violent thermonuclear runaway ensues. Classical novae are believed to be major sources of the Galactic 15N, 17O and 13C, with a minor contribution on a number of additional species, mainly 7Li and 26Al. But there are reasons to believe that these nucleosynthetic features have varied during the overall 10 Gyr of Galactic history. In this paper, we review recent progress on the characterization of primordial novae, that is, novae exploding in primordial cataclysmic binaries, and will compare their expected nucleosynthetic pattern with that of classical novae. Emphasis is made on the dominant nuclear paths during the explosion and on a thorough comparison with other explosive sites, such as type-I x-ray bursts. [ABSTRACT FROM AUTHOR]