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Astron. Astrophys. 364, L75-L79 (2000)

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1. Introduction

The recurrent nova T Pyx is the most prolific of all recurrent novae, with a mean outburst recurrence time of only 22 years. But even away from outburst, T Pyx's observational characteristics are remarkable. Most importantly, its quiescent bolometric luminosity is [FORMULA] ergs s-1 (Patterson et al. 1998). This is much higher than expected for a short-period cataclysmic variable (CV) like T Pyx, whose orbital period of [FORMULA] hr places it well below the period gap.

The implied disagreement with theoretical expectations is severe. For reasonable estimates of T Pyx's system parameters, the standard model of CV evolution predicts mass transfer to be driven by gravitational radiation at a rate of [FORMULA]. The corresponding accretion luminosity is only [FORMULA] erg s-1. Thus the gap between T Pyx's observed luminosity and the theoretically predicted value spans at least three orders of magnitude.

In order to account for T Pyx's extreme luminosity without having to postulate a similarly extreme mass-transfer rate for the system, it has been suggested that, even in quiescence, the system may be powered predominantly by nuclear burning on the surface of the white dwarf (WD), rather than by accretion (e.g. Webbink et al. 1987; Patterson et al. 1998). However, even this can only reduce the luminosity gap by about an order of magnitude (c.f. Sect. 2.2). Thus regardless of the actual mode of radiation energy release, T Pyx's extreme luminosity implies an accretion rate [FORMULA]. This lower limit is in line with estimates based on theoretical nova models: in order to reproduce T Pyx's short outburst recurrence time, an accretion rate [FORMULA] appears to be required (Livio & Truran 1992; Prialnik & Kovetz 1995; Contini & Prialnik 1997).

Clearly, T Pyx poses a fundamental challenge to our understanding of CV evolution. Put concisely, the question is this: why is T Pyx's accretion rate so abnormally high relative to "ordinary" CVs? The answer we propose in this Letter is that T Pyx has left the standard CV evolutionary track completely and is instead currently evolving as a wind-driven supersoft X-ray source.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001