On the Nature of the Hot Component in the Symbiotic, Supersoft X-ray Binary AG Draconis

TL;DR

This study models the hot component of AG Draconis, revealing it is likely a hot white dwarf with minimal disk contribution, suggesting soft X-rays originate from a boundary layer rather than an accretion disk.

Contribution

The paper introduces NLTE spectral models to analyze AG Dra's FUV spectrum, providing new insights into the nature of its hot component and its distance.

Findings

Best fit is a white dwarf with Teff=80,000K and mass=0.4 Msun

Distance estimated at approximately 1543 parsecs

Accretion disk contributes minimally to FUV flux

Abstract

AG Dra is a symbiotic variable consisting of a metal poor, yellow giant mass donor under-filling its Roche lobe, and a hot accreting white dwarf, possibly surrounded by an optically thick, bright accretion disk which could be present from wind accretion. We constructed NLTE synthetic spectral models for white dwarf spectra and optically thick accretion disk spectra to model a FUSE spectrum of AG Dra, obtained when the hot component is viewed in front of the yellow giant. The spectrum has been de-reddened (E(B-V) = 0.05) and the model fitting carried out, with the distance regarded as a free parameter, but required to be larger than the Hipparcos lower limit of 1 kpc. We find that the best-fitting model is a bare accreting white dwarf with Mwd = 0.4 Msun, Teff = 80,000K and a model-derived distance of 1543 pc. Higher temperatures are ruled out due to excess flux at the shortest wavelengths while a lower temperature decreases the distance below 1 kpc. Any accretion disk which might be present is a only a minor contributor to the FUV flux. This raises the possibility that the soft X-rays originate from a very hot boundary layer between a putative accretion disk and the accreting star.