The peculiar X-ray behaviour of the outbursting symbiotic star AG draconis

Abstract

The symbiotic system AG Dra is one of the best representatives of the category of the supersoft X-ray sources. The system was subject in recent times to a sequence of visual outbursts which were monitored in X-rays with the EXOSAT and ROSAT satellites, and in the ultraviolet with the IUE satellite. We find that during all the outbursts so far monitored, the visual brightening was associated with an increase of the UV flux, while the X-ray countrate largely faded. This systematic optical/UV and X-ray anticorrelation is explained, rather than by a quencing of the high energy source, by the increase of the accretor radius during outburst, and by the consequent decrease of its surface temperature which moves the maximum emission from soft X-rays to EUV.