X-ray observations of classical novae. Theoretical implications

arXiv:0910.4607v1 [astro-ph.HE] 24 Oct 2009 Astron. Nachr. / AN x, No. x, 1 – 6 (2009) / DOI please set DOI! X-ray observations of classical novae. Theoretical implications M. Hernanz1,⋆and G. Sala2 1 Institut de Ci`encies de l’Espai (CSIC-IEEC), Campus UAB, Facultat de Ci`encies, C5 parell 2on, 08193 Bellaterra (Barcelona), Spain 2 Departament F´ısica i Enginyeria Nuclear, EUETIB (UPC-IEEC), Comte d’Urgell 187, 08036 Barcelona, Spain The dates of receipt and acceptance should be inserted later Key words stars: novae, cataclysmic variables – stars: white dwarfs – X-rays: binaries Detection of X-rays from classical novae, both in outburst and post-outburst, provides unique and crucial information about the explosion mechanism. Soft X-rays reveal the hot white dwarf photosphere, whenever hydrogen (H) nuclear burning is still on and expanding envelope is transparent enough, whereas harder X-rays give information about the ejecta and/or the accretion flow in the reborn cataclysmic variable. The duration of the supersoft X-ray emission phase is related to the turn-off of the classical nova, i.e., of the H-burning on top of the white dwarf core. A review of X-ray observations is presented, with a special emphasis on the implications for the duration of post-outburst steady H-burning and its theoretical explanation. The particular case of recurrent novae (both the ”standard” objects and the recently discovered ones) will also be reviewed, in terms of theoretical feasibility of short recurrence periods, as well as regarding implications for scenarios of type Ia supernovae. c ⃝2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction White dwarfs in cataclysmic variables explode as classical novae, when they accrete H-rich matter from their main se- quence companion with some particular combinations of mass-accretion rate and initial white dwarf mass and lumi- nosity. Matter accumulates on top of the white dwarf until hydrogen ignites, under degenerate or semi-degenerate con- ditions.This leads to a thermonuclear runaway, because self adjustment of the envelope once nuclear burning starts is not possible. As a consequence of the explosion, a fraction of the accreted envelope (or even the whole envelope plus some material dredged-up from the white dwarf core) is ejected, with mass typically in the range ∼10−6−10−4 M⊙, veloc- ity of 100’s or 1000’s of km/s, and luminosity up to around ∼104−5 L⊙(Prialnik & Kovetz 1995, Starrfield et al. 1998, Jos´e & Hernanz 1998). Steady nuclear burning is expected to take place in the remaining (if existing) H-rich envelope on top of the white dwarf. In fact, multiwavelength obser- vations of classical novae have shown that a phase of con- stant bolometric luminosity (with values close to the Ed- dington limit) occurs after maximum, during the early op- tical decline; as the envelope mass is depleted, the pho- tospheric radius decreases and thus the effective tempera- ture increases, leading to a hardening of the spectrum, from optical to ultraviolet, extreme ultraviolet and soft X-rays, in agreement with theoretical predictions (Starrfield 1989, MacDonald 1996, Krautter 2002). Therefore, novae are ex- pected to emit soft X-rays (E≤1 keV) when steady H-bur- ⋆Corresponding author: e-mail: hernanz@ieec.uab.es ning at the base of the white dwarf (WD) envelope heats the WD photosphere to effective temperatures from a few 105 K up to 106 K (Kato & Hachisu 1994, Sala & Hernanz 2005a, Kato these proceedings), provided that the expanding enve- lope is not opaque to X-rays; the nova then behaves as a su- persoft source, SSS. However, if no H-rich envelope is left, or if the steady H-burning phase ends-up before the ejecta becomes optically thin to soft X-rays, the SSS phase will be absent. But such very short duration steady H-burning phases correspond to very tiny H-rich envelopes left after the explosion, much smaller than predicted by hydrodynam- ical models of nova explosions. From the observation of the soft X-ray emission, the du- ration of the turn-off of the nova explosion can be measured; some other properties, such as the mass and metallicitof the remnant H-burning envelope and the mass of the underlying WD can be constrained, by comparing observations with current post-outburst nova models of this phase (Sala & Hernanz 2005a). Additional information about the decline of classical novae comes from UV observations (Shore et al. 1996, G´onzalez-Riestra et al. 1998): turn-off times de- rived from UV data were in agreement with the soft X-ray ones, when both data were available. There are other mechanisms of X-ray emission in clas- sical novae, not related to residual H-nuclear burning on top of the white dwarf. Internal shocks in the expanding enve- lope, as well as shocks between the ejecta and circumstellar matter, lead to the heating of the plasma and the ensuing emission of X-rays, mainly by thermal bremsstrahlung; the corresponding energy of the emitted photons is larger than

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