For the first time a RN (U Sco) has been detected in a SSS X-ray phase with the BeppoSAX X-ray satellite 20 days after an optical outburst. This observation confirms the theoretical predictions that RN have a SSS X-ray phase (Yungelson et al. 1996; Kato 1996). He enhanced non-LTE WD atmosphere model spectra, with a high N/C ratio (using abundances derived from the ejecta) are required to fit the BeppoSAX X-ray spectrum of U Sco. This is evidence that the outburst of U Sco was triggered by a TR and that the CNO cycle was active. From the temperature of the optically thick SSS component of , we constrain the WD to be very massive () and consistent to be close to the CH limit.
Besides the SSS emission we observe an additional optically thin component. We explain this hard component as emission from a strong shocked wind from the WD with a mass loss rate of . Such a component is consistent with the theoretical predictions for a WD with a mass just below the CH mass (Hachisu et al. 1999). According to their calculations U Sco emerged from an optically thick wind phase when the BeppoSAX observation was performed and it cannot last longer than 20 days for a mass just below the CH limit.
U Sco, and therefore RN in general, can be considered to be progenitors of SN Ia. The condition that the WD can grow in mass is achieved if the accreted and accumulated material is enriched in He and not all the envelope was ejected. This condition may occur if the donor star has experienced a previous helium accretion phase, if it is somewhat evolved (a subgiant), or if helium rich material has been mixed into the accreted envelope.
© European Southern Observatory (ESO) 1999
Online publication: June 6, 1999