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Astron. Astrophys. 322, 576-590 (1997)

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6. Summary

The major results and conclusions of the present paper can be summarised as follows:

  • The X-ray spectrum in quiescence is very soft, with a blackbody temperature of about 14.5 [FORMULA] 1 eV.
  • The quiescent bolometric luminosity of the hot component in the AG Dra binary system is (9.5 [FORMULA] 1.5) [FORMULA] 1036 (D/2.5 kpc)2 erg/s, thus suggesting stable surface hydrogen burning in quiescence. Adopting a distance of AG Dra of 2.5 kpc, the X-ray luminosity suggests a low-mass white dwarf (M [FORMULA] 0.6 [FORMULA]).
  • In order to sustain the high luminosity the cool companion in AG Dra either has to have a wind mass loss substantially larger than usual K giants or is required to fill its Roche lobe, and consequently has to be brighter than a usual K giant. The mass of the cool component should be smaller than [FORMULA] 2 [FORMULA] for the given orbital parameters and the mass ratio would be 2-5.
  • The monitoring of AG Dra at X-rays and UV wavelengths did not yield any hints for the predicted eclipse during the times of the U light minima.
  • The recent optical outbursts of AG Dra have given us the rare opportunity to study the rapid evolution of the X-ray and UV emission with time. During the optical outburst in 1994 the UV continuum increased by a factor of 10, the UV line intensity by a factor of 2 (see Fig. 2), and the X-ray intensity dropped by at least a factor of 100 (see Fig. 3). There is no substantial time lag between the variations in the different energy bands compared to the optical variations.
  • There is no hint for an increase of the absorbing column during the one ROSAT PSPC X-ray observation (with spectral resolution) performed during the decline phase of the 1994 optical outburst. Instead, a temperature decrease is consistent with the X-ray data which is also supported by the IUE spectral results.
  • Modelling the X-ray intensity drop by a slowly expanding white dwarf with concordant cooling, we find that the accretion rate has to rise only slightly above [FORMULA]. Accordingly, the white dwarf expands to approximately its double size within the about three months rise of the optical outburst. The cooling during this expansion is moderate: the temperature decreases by only about 35%.
  • The UV continuum emission during quiescence consists of two components which both match perfectly to the neighbouring wavelength regions (see Fig. 5). Shortwards of [FORMULA] 2000Å it corresponds to the tail of the 14-15 eV hot component as derived from the X-ray observations. However, during the optical outbursts the UV continuum shortwards of [FORMULA] 2000Å does not correspond to the tail of the cooling hot component (9-11 eV), but instead is completely dominated by another emission mechanism, possibly a recombination continuum in the wind zone around the hot component.
  • AG Dra could either be a symbiotic nova for which the turn on would have occurred before 1855, or the first example of the wide-binary supersoft source class.
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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998