Multiwavelength modelling the SED of supersoft X-ray sources. I. The method and examples

Radiation of supersoft X-ray sources (SSS) dominates both the supersof X-ray and the far-UV domain. A fraction of their radiation can be reprocessed into the thermal nebular emission, seen in the spectrum from the near-UV to longer wavelengths. In the case of symbiotic X-ray binaries (SyXBs) a strong contribution from their cool giants is indicated in the optical/near-IR. In this paper I introduce a method of multiwavelength modelling the spectral energy distribution (SED) of SSSs from the supersoft X-rays to the near-IR with the aim to determine the physical parameters of their composite spectra. The method is demonstrated on two extragalactic SSSs, the SyXB RX J0059.1-7505 (LIN 358) in the Small Magellanic Cloud (SMC), RX J0439.8-6809 in the Large Magellanic Cloud (LMC) and two Galactic SSSs, the classical nova RX J2030.5+5237 (V1974 Cyg) during its supersoft phase and the classical symbiotic star RX J1601.6+6648 (AG Dra) during its quiescent phase. The multiwavelength approach overcomes the problem of the mutual dependence between the temperature, luminosity and amount of absorption, which appears when only the X-ray data are fitted.Thus, the method provides an unambiguous solution. It was found that selection of the model (a blackbody or an atmospheric model) is not of crucial importance in fitting the global X-ray/IR SED. The multiwavelength modelling of the SED of SSSs is essential in determining their physical parameters.Comment: 15 pages, 11 figures, 2 tables, accepted for New Astronom

The applicability of the wind compression model

Compression of the stellar winds from rapidly rotating hot stars is described by the wind compression model. However, it was also shown that rapid rotation leads to rotational distortion of the stellar surface, resulting in the appearance of non-radial forces acting against the wind compression. In this note we justify the wind compression model for moderately rotating white dwarfs and slowly rotating giants. The former could be conducive to understanding density/ionization structure of the mass outflow from symbiotic stars and novae, while the latter can represent an effective mass-transfer mode in the wide interacting binaries.Comment: 3 pages, A&

Wind asymmetry imprint in the UV light curves of the symbiotic binary SY Mus

Context: Light curves (LCs) of some symbiotic stars show a different slope of the ascending and descending branch of their minimum profile. The origin of this asymmetry is not understood well. Aims: We explain this effect in the ultraviolet LCs of the symbiotic binary SY Mus. Methods: We model the continuum fluxes in the spectra obtained by the International Ultraviolet Explorer at 10 wavelengths, from 1280 to 3080 \AA. We consider that the white dwarf radiation is attenuated by $H^0$ atoms, $H^-$ ions and free electrons in the red giant wind. Variation in the nebular component is approximated by a sine wave along the orbit as suggested by spectral energy distribution models. The model includes asymmetric wind velocity distribution and the corresponding ionization structure of the binary. Results: We determined distribution of the $H^0$ and $H^+$, as well as upper limits of $H^-$ and $H^0$ column densities in the neutral and ionized region at the selected wavelengths as functions of the orbital phase. Corresponding models of the LCs match well the observed continuum fluxes. In this way, we suggested the main UV continuum absorbing (scattering) processes in the circumbinary environment of S-type symbiotic stars. Conclusions: The asymmetric profile of the ultraviolet LCs of SY Mus is caused by the asymmetric distribution of the circumstellar matter at the near-orbital-plane area.Comment: 9 pages, 9 figure

Ionization structure of hot components in symbiotic binaries during active phases

During active phases of symbiotic binaries, an optically thick medium in the form of a flared disk develops around their hot stars. During quiescent phases, this structure is not evident. We propose the formation of a flared neutral disk-like structure around the rotating white dwarf (WD) in symbiotic binaries. We applied the wind compression model and calculated the ionization boundaries in the compressed wind from the WD using the equation of photoionization equilibrium. During active phases, the compression of the enhanced wind from the rotating WD can form a neutral disk-like zone at the equatorial plane, while the remainder of the sphere above/below the disk is ionized. Calculated hydrogen column density throughout the neutral zone and the emission measure of the ionized fraction of the wind are consistent with those derived from observations. During quiescent phases, the neutral disk-like structure cannot be created because of insufficient mass loss rate. Formation of the neutral disk-like zone at the equatorial plane is connected with the enhanced wind from the rotating WD, observed during active phases of symbiotic binaries. This probably represents a common origin of warm pseudophotospheres, indicated in the spectrum of active symbiotic binaries with a high orbital inclination.Comment: 10 pages, 6 figures, accepted for publication in Astronomy and Astrophysic