The UV/X-ray emission of the symbiotic star AG Draconis during quiescence and the 1994/1995 outbursts ?

We present the results of an extensive campaign of coordinated X-ray (ROSAT) and UV (IUE) observations of the symbiotic star AG Dra during a long period of quiescence followed recently by a remarkable phase of activity character- izedbytwoopticaloutbursts.ThemajoropticaloutburstinJune 1994andthesecondaryoutburstinJuly1995werecoveredbya number of target of opportunity observations (TOO) with both satellites. Optical photometry is used to establish the state of evolution along the outburst. Ouroutburstobservationsaresupplementedbyasubstantial number of X-ray observations of AG Dra during its quiescent phase between 1990{1993. Near-simultaneous IUE observa- tions at the end of 1992 are used to derive the spectral energy distribution from the optical to the X-ray range. The X-ray flux remained constant over this three year quiescent phase. The hot component (i.e. X-ray emitting compact object) turns out to be very luminous: a blackbody t to the X-ray data in qui- escence with an absorbing column equal to the total galactic NH in this direction gives (9.51.5)10 36 (D/2.5 kpc) 2 erg/s. This suggests that the compact object is burning hydrogen-rich matter on its surface even in the quiescent (as dened optically) state at a rate of (3.20.5)10 8 (D/2.5 kpc) 2 M/yr. Assum- ing a steady state, i.e. burning at precisely the accretion supply rate, this high rate suggests a Roche lobe lling cool compan- ion though Bondi-Hoyle accretion from the companion wind cannot be excluded. With ROSAT observations we have discovered a remark- able decrease of the X-ray flux during both optical maxima, followed by a gradual recovering to the pre-outburst flux. In the UV these events were characterized by a large increase of