Astron. Astrophys. 347, 478-493 (1999)

6. Conclusions

The multifrequency study of AG Dra during quiescence and activity has provided a wealth of information which is fundamental for understanding its nature. It is outside the scope of the present work to perform a full interpretation of the data, for which it would be necessary to better know some of the fundamental parameters of the system (distance, separation, radius and luminosity of the cool star). Here we were mostly concerned on the general behaviour during quiescence and outburst. We summarize our main results as follows:

• During quiescence the flux of the higher ionization emission lines and of the near-UV continuum is modulated according to the 554 days orbital motion, suggesting emission from a region placed in the line connecting the two stars and closer to the white dwarf. This region should be extended because of the broadness of the minimum. A contribution to these variation of the heated side of the K-giant facing the WD cannot be excluded, and would not be in contradiction with the relative phasing of the radial velocity and u-band light curves (the WD is in front of the giant at the time of the U maximum, Mikolajewska et al. 1995). However, one of us (TI) has not detected any significant change of the optical photospheric lines along the orbit during quiescence.

• The far-UV flux shows little modulation. The absence of eclipses in this range is suggestive of a not large orbital inclination, in agreement with the non existence of eclipses in X-rays. This result also agrees with a model of X-rays formed on or close to the WD surface.

• The orbit-to-orbit variation of the UV fluxes, in contrast to the stability of the X-ray flux during quiescence, could be related to irregular fluctuation of the density of the cool star wind.

• During the different outbursts AG Dra displayed a variety of behaviours, which cannot be fully described due to the scarcity of observations during the early stages of the outbursts. We can make a gross subdivision of the bursts into cooler (, , , ) and hotter bursts (, ), depending on whether the He II Zanstra temperature is smaller or larger than in quiescence. It must be noted that the "hot" bursts were much weaker in the UV and optical continuum than the "cool" ones.

• The decay time of the weak "hot" 1985-86 outbursts was substantially shorter than in the other active phases. While the system had not fully returned to quiescence when the and outbursts started (approximately one year after the and peaks), quiescence levels where reached only 70 days after the maximum.

• The beginning of the 1994 outburst was marked by the appearance of strong P Cygni absorptions in the high ionization resonance lines, with terminal velocities up to 700 km s^-1, never observed before in this object.

  These features might significantly affect the emission line flux observed at low resolution during a critical phase of AG Dra, and lead to wrong interpretations.

• The strong anticorrelation between optical/UV and X-rays during the "cool" outbursts is due to the expansion and consequent cooling of the white dwarf. The temperature of the white dwarf decreases by 10-25%, while its radius expands by factors between 2 and 6, thus causing the observed large brightening of AG Dra. The smaller amplitude "hot" outbursts are due to an increase of the stellar temperature. The onset of an extended atmosphere and of an opaque hot wind would produce a high energy cutoff which would drastically reduce the soft-X ray flux in the EXOSAT sensitivity range, as observed.

© European Southern Observatory (ESO) 1999

Online publication: June 30, 1999
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