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Astron. Astrophys. 347, L43-L46 (1999)

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3. Discussion

3.1. Supersoft X-ray emission

U Sco belongs now to those novae for which SSS X-ray emission has been discovered (cf. Orio & Greiner 1999).

The TR theory predicts two processes which can generate soft X-rays: Shock acceleration in the nova ejecta and steady nuclear burning. For Nova Cyg 1992 an optically thin component due to shocks has been detected. In addition an optically thick SSS X-ray component has been observed [FORMULA]60 days after the outburst and for [FORMULA]600 days (Krautter et al. 1996; Balman et al. 1998).

According to the calculations of Kato (1996) performed for U Sco, and assuming a massive ([FORMULA]=[FORMULA]) WD a SSS component is predicted to be observed from [FORMULA]10 days after the outburst. In the case of the H-rich model (He/H=0.1) the supersoft component is expected to rise till [FORMULA]50 days after the outburst to a maximum luminosity of [FORMULA]. In the He-rich model (He/H=2), a maximum luminosity for the SSS component of [FORMULA] is reached [FORMULA]20 days after the optical outburst. Using the He enriched fit with the N/C ratio enhanced (Table 1), the observed bolometric luminosity [FORMULA]19-20 days after the optical outburst is [FORMULA]. Assuming a distance [FORMULA]14 kpc (see Introduction) a bolometric luminosity of [FORMULA] is derived. The luminosity is in agreement with the bolometric luminosity of [FORMULA] predicted for novae (Mc Donald et al. 1985). The temperature of [FORMULA] and the luminosity of [FORMULA] derived from the X-ray spectral fit requires a very massive [FORMULA] WD consistent with an almost CH mass WD (e.g. Kato 1997).


Table 1. Best-fit values derived from spectral fits to the BeppoSAX LECS and MECS spectrum of U Sco using (a) a blackbody model with absorption edges and (b) an optically thick non-LTE WD atmosphere model (with He enriched and the N/C ratio enhanced) and an optically thin Raymond and Smith component (assuming He enriched and the ratio N/C enhanced). 90% confidence parameter ranges are given. For the edges the absorption depth at the given energies are listed

3.2. Spectrally hard component

In addition to the optically thick SSS X-ray model spectrum, the spectral fits require a spectrally hard component. A similar component in addition to a SSS component was used by Balman et al. (1998) for X-ray spectral fits to the classical nova Cyg 1992. Using an optically thin thermal model we derive a temperature of [FORMULA], an emission measure [FORMULA] if He is enriched and the ratio N/C enhanced.

If we assume a terminal wind velocity of [FORMULA] the wind mass loss rate for a He/H=2 mixture can be estimated from [FORMULA]. Here r is the typical radius of the emitting region. We assume [FORMULA], the radius of the Roche-lobe, and use the result of the spectral fit assuming He is enriched. We then obtain a wind mass loss rate of [FORMULA]. For a distance to U Sco of 14 kpc, we derive a wind mass loss rate of [FORMULA]. A near-CH mass WD at [FORMULA] experiences an envelope mass loss of [FORMULA] due to both steady nuclear burning and a wind from the WD. The steady nuclear burning mass loss can be estimated to be [FORMULA] (Hachisu et al. 1999). The mass loss due to the wind is [FORMULA]. This value is in agreement with the range we derived above. For a duration of the steady nuclear burning phase plus wind mass loss phase of [FORMULA]0.1 year (Kato 1996) we derive a mass loss from the WD envelope of [FORMULA] of which [FORMULA] is due to the wind. In addition the predicted post-outburst envelope mass is [FORMULA] (Hachisu et al. 1999). This would mean that 70% of the envelope mass has remained on the WD allowing it to increase in mass. Williams et al. (1981) derive from the UV lines (for 14 kpc and [FORMULA]) a wind mass loss rate [FORMULA] which differs significantly from our value, although it is subject to many uncertainties, and differences between outbursts cannot be accounted for.

If the helium fraction is indeed large ([FORMULA]) only part of the accreted He/H envelope might have been ejected and steady nuclear burning proceeded for at least one month. This result is consistent with the analytical model of Kahabka (1995). Assuming an X-ray on-time of 0.1 years and a recurrence period of 10 years, we constrain the WD mass to [FORMULA]. U Sco and RN in general are therefore probably SN Ia progenitors (cf. Li & van den Heuvel 1997; for a recent review on SN Ia, see Livio 1999).

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© European Southern Observatory (ESO) 1999

Online publication: June 6, 1999