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

¹ Astronomical Institute, Slovak Academy of Sciences, 059 60 Tatranská Lomnica, Slovakia e-mail: skopal@astro.sk
² XMM Science Operations Centre, ESAC, PO Box 78, 28691 Villanueva de la Cañada, Madrid, Spain
³ INAF Instituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy

Received: 25 November 2008
Accepted: 27 August 2009

Abstract

Context. AG Draconis produces a strong supersoft X-ray emission. The X-ray and optical/UV fluxes are in strict anticorrelation throughout 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 IUE spectra, and optical/near-IR spectroscopic and/or photometric observations. We modeled the spectral energy distribution and broad wings of the O vi λ1032, λ1038 and He ii λ1640 lines by the electron-scattering during the maximum of the 2003 burst, and the subsequent transition and quiescent phase.

Results. The X-ray-near-IR energy distribution at different levels of the star's brightness confirmed the observed flux anticorrelation quantitatively and showed that the optical bursts are associated to an increase in the nebular component of radiation. The profile-fitting analysis revealed a significant increase in the mean particle density around the hot star from ~2.610¹⁰ cm^-3 during quiescent phase to ~1.110¹² cm^-3 during the burst.

Conclusions. The supersoft X-ray emission is produced by the white dwarf photosphere. The X-ray and far-UV fluxes make it possible to determine its temperature unambiguously. 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.