On the Nature of the Hot Component in the Symbiotic, Supersoft X-Ray Binary Ag Draconis

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 1 Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE) was operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985.2 Visiting at the Henry Rowland Department of Physics, The Johns Hopkins University, Baltimore, MD.-2 -be larger than the Hipparcos lower limit of 1 kpc. We find that the best-fitting model is a bare accreting white dwarf with M wd = 0.4M ⊙ , T ef f = 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.

Spectroscopic view on the outburst activity of the symbiotic binary AG Draconis

“…Iijima (1981) has provided a simple formula for assessment of the effective temperature of a central source of ionizing photons from the nebular emission-line fluxes: (2) Equation ( 2) yields a temperature of the hot component at about 170 000 K for the average line ratio of 0.70 as well as maximum (210 000 K) and minimum (108 000 K) values for the extrema 1.26 and 0.16, respectively. These temperatures are somewhat higher than those derived by González-Riestra et al (1999), and in particular that by Sion et al (2012). We are aware that by neglecting the He i line the temperature increases.…”

“…The hot component is a white dwarf with an effective temperature of 100 000 K and a luminosity of ∼ 10 3 L (Miko lajewska et al 1995) obtained from the IUE data. However, Sion et al (2012) have derived T eff = 80 000 K from the FUSE far-UV spectrum. The orbital period of the binary system is 550 d (Meinunger 1979;Smith et al 1996;Fekel et al 2000).…”

Discovery of optical flickering from the symbiotic star EF Aquilae

“…In Galaxy Evolution Explorer (GALEX) catalogs of ultraviolet (UV) sources (Bianchi et al ), we find FUV and NUV magnitudes of EF Aql (16.797, 16.269), T CrB (16.166, 15.227), RS Oph (16.284, 15.841), AG Dra (12.003, 12.580), and RR Tel (12.428, 13.229). For RS Oph, we adopt E ( B − V )=0.60 (e.g., Nelson et al ) and a distance of 1,400 pc (Barry et al ); for T CrB— E ( B − V )=0.10 and a distance of 1,020 pc (Harrison et al ); for AG Dra— E ( B − V )=0.05, d =2,500 pc (Sion et al ); and for RR Tel— E ( B − V )=0.10 (Young et al ), d =2,500 pc (Kotnik‐Karuza et al ). For the interstellar extinction, we apply A FUV = 8.29× E ( B − V ) and A NUV = 8.18× E ( B − V ) (Seibert et al ).…”

Recent outburst activity of the symbiotic binary AG Draconis

“…This binary system consists of a metal-poor red giant of spectral type K3 III (Shenavrin et al, 2011) with mass of 1.5 M (Kenyon & Fernandez-Castro, 1987). The giant is under-filling its Roche lobe (Sion et al, 2012) and the accretion most likely takes place by the stellar wind from the cool giant.…”