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

Abstract

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 (1990-1993) followed by two optical outbursts in 1994 and 1995. The hot component (i.e. X-ray emitting compact object) turns out to be very luminous during the quiescent phase: (9.5+/-1.5)*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 defined optically) state at a rate of (3.2+/-0.5)*10^-8 (D/2.5 kpc)^2 M_sun/yr. Assuming a steady state, i.e. burning at precisely the accretion supply rate, this high rate suggests a Roche lobe filling cool companion though Bondi-Hoyle accretion from the companion wind cannot be excluded. We have discovered a remarkable decrease of the X-ray flux during both optical maxima. In the UV these events were characterized by a large increase of the emission line and continuum fluxes. The anticorrelation of X-ray/UV flux and optical brightness evolution is very likely due to a temperature decrease of the hot component which could be the result of an increased mass transfer to the burning compact object, causing it to slowly expand to about twice its original size during each optical outburst.