X-RAY FLASHES IN RECURRENT NOVAE: M31N 2008-12a AND THE IMPLICATIONS OF THE SWIFT NONDETECTION

• Published in: The Astrophysical journal Vol. 830; no. 1; pp. 40 - 51
• Main Authors: Kato, Mariko, Saio, Hideyuki, Henze, Martin, Ness, Jan-Uwe, Osborne, Julian P., Page, Kim L., Darnley, Matthew J., Bode, Michael F., Shafter, Allen W., Hernanz, Margarita, Gehrels, Neil, Kennea, Jamie, Hachisu, Izumi
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 10.10.2016; IOP Publishing
• Subjects: Astrophysics; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; DETECTION; Dwarf novae; EMISSION; EXPLOSIONS; HYDROGEN; IGNITION; Light curve; NOVAE; novae, cataclysmic variables; Outbursts; stars: individual (M31N 2008-12a); WHITE DWARF STARS; white dwarfs; X ray flashes; X-ray emissions; X-rays: binaries
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/0004-637X/830/1/40

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 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.