The origin of the supersoft X-ray-optical/UV flux anticorrelation in the symbiotic binary AG?Draconis

摘要

Context. AGDraconis produces a strong supersoft X-rayemission. The X-ray and optical/UV fluxes are in strict anticorrelationthroughout the active and quiescent phases. Aims. We identify the source of the X-ray emission and reveal the nature of the observed flux anticorrelation. Methods. We used X-ray and UV observations with XMM-Newton, far-UV spectroscopy from FUSE, low- and high-resolution IUEspectra,and opticalear-IR spectroscopic and/or photometric observations. Wemodeled the spectral energy distribution and broad wings of the O VIand He II1640linesby the electron-scattering during the maximum of the 2003 burst, andthe subsequent transition and quiescent phase. Results. The X-ray-near-IR energy distribution at differentlevels of the star's brightness confirmed the observed fluxanticorrelation quantitatively and showed that the optical bursts areassociated to an increase in the nebular component of radiation. Theprofile-fitting analysis revealed a significant increase in the meanparticle density around the hot star from cm-3 during quiescent phase to cm-3 during the burst. Conclusions. The supersoft X-ray emission is produced by thewhite dwarf photosphere. The X-ray and far-UV fluxes make it possibleto determine its temperature unambiguously. The supersoftX-ray-optical/UV flux anticorrelation is caused by the variable windfrom the hot star. The enhanced hot star wind gives rise to the opticalbursts by reprocessing high-energy photons from the Lyman continuum tothe optical/UV. Key words: stars: binaries: symbiotic - stars: fundamental parameters - X-rays: binaries - X-rays: individuals: AGDraconis