The origin of the supersoft X-ray/optical-UV flux anticorrelation in the symbiotic binary AG Draconis
Authors:
A. Skopal, M. Sekeras, R. Gonzalez-Riestra, R. Viotti
Image & caption:
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Image caption::
Left panels show a comparison of the observed and modeled SEDs of AG Dra during the hot burst (top), transition phase (middle), and quiescent phase (bottom). Fluxes are in units of erg/s/cm^2/A. Right panels compare the observed and modeled broad wings of the OVI 1032, 1038 A doublet and the HeII 1640 A line at these stages of activity.
Description:
AG Draconis produces a strong supersoft X-ray emission.The X-ray and optical-UV fluxes are in a strict anticorrelation throughout the active and quiescent phases. To understand this relationship we modeled the X-ray/near-IR energy distribution at different levels of the star's brightness, and provided a profile-fitting analysis of the broad wings of OVI 1032,1038 A and HeII 1640 A emission lines by Thomson scattering. By this way we confirmed the observed flux anticorrelation quantitatively, and showed that the optical bursts are associated to an increase in the nebular component of radiation. These results led us to a conclusion that the supersoft X-ray/optical-UV flux
anticorrelation is caused by the variable wind from the hot star. The enhanced hot star wind gives rise to the optical bursts by reprocessing high-energy photons from the Lyman continuum to the optical-UV. Understanding the inverse relationship between optical and X-ray fluxes represents an important ingredient in the investigation of the Z And-type outbursts.
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