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

TL;DR

This paper investigates the source of supersoft X-ray emission in AG Draconis and explains the observed flux anticorrelation with optical/UV light as caused by variable stellar wind reprocessing high-energy photons.

Contribution

It identifies the white dwarf photosphere as the X-ray source and explains the flux anticorrelation through wind-driven reprocessing of high-energy photons.

Findings

X-ray emission originates from the white dwarf photosphere.

Flux anticorrelation driven by variable wind from the hot star.

Wind enhances optical bursts by reprocessing high-energy photons.

Abstract

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. The aim of this contribution is to identify the source of the X-ray emission and reveal the nature of the observed flux anticorrelation. For this purpose we model the 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. Our analysis showed that 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.