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

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

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 1 eV.
The quiescent bolometric luminosity of the hot component in the AG Dra binary system is (9.5 1.5) 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 0.6 ).
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 2 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 . 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 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 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.

Online publication: June 5, 1998

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