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

TL;DR

This paper models the expected X-ray flash in recurrent novae, predicts its short duration, and reports the first observational attempt with Swift that did not detect the flash, informing nova outburst theories.

Contribution

It provides the first theoretical light curves of X-ray flashes for very massive white dwarfs and reports the initial observational search for such flashes in M31N 2008-12a.

Findings

X-ray flash durations are very short, as brief as 14 hours for certain white dwarf masses.

The Swift observation did not detect X-ray flashes prior to optical detection.

Short recurrence period novae may produce observable X-ray flashes, aiding in understanding nova physics.

Abstract

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 M_sun and for two recurrence periods of 0.5 and 1 years. 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 hours (0.6 days) among the four cases is obtained for the 1.385 M_sun WD with one year 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 year recurrence period), which resulted in the non-detection 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 essentially important to understand the pre-optical-maximum phase. We encourage further observations.