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Models of nova outbursts suggest that an X-ray flash should occur just afterhydrogen ignition. However, this X-ray flash has never been observationallyconfirmed. We present four theoretical light curves of the X-ray flash for twovery massive white dwarfs (WDs) of 1.380 and 1.385 M_sun and for two recurrenceperiods of 0.5 and 1 years. The duration of the X-ray flash is shorter for amore massive WD and for a longer recurrence period. The shortest duration of 14hours (0.6 days) among the four cases is obtained for the 1.385 M_sun WD withone year recurrence period. In general, a nova explosion is relatively weak fora very short recurrence period, which results in a rather slow evolution towardthe optical peak. This slow timescale and the predictability of very shortrecurrence period novae give us a chance to observe X-ray flashes of recurrentnovae. In this context, we report the first attempt, using the Swiftobservatory, to detect an X-ray flash of the recurrent nova M31N 2008-12a (0.5or 1 year recurrence period), which resulted in the non-detection of X-rayemission during the period of 8 days before the optical detection. We discussthe impact of these observations on nova outburst theory. The X-ray flash isone of the last frontiers of nova studies and its detection is essentiallyimportant to understand the pre-optical-maximum phase. We encourage furtherobservations.

X-Ray Flashes in Recurrent Novae: M31N 2008-12a and the Implications of the Swift Non-detection