Unified model fitting to variable X-ray spectra of Cygnus X-3

Abstract

The Japanese X-ray astronomy satellite Ginga has observed Cyg X-3 in high- and low-intensity states. The X-ray spectrum of Cyg X-3 is so variable and complex that no unified model which can well explain all the observed spectra has yet been proposed. Now we have achieved a model functional form which is well fitted to X -ray spectra in most cases. Most parameters of the present ‘unified model’ have been proposed in previous papers, but an essential point of this model is a successful combination of physical parameters. The model consists of three components: (1) an absorbed power-law component modified with an exponential cut-off at high energy, (2) an absorbed blackbody component and (3) an iron emission line. The model fitting results in the hydrogen column density representing the absorption for the power-law component being thinner than that for the blackbody component. In addition, the observed spectra require an absorption edge at an energy of about 9 ke V. In the fitting of our model to the Ginga data, the reduced χ2 value is less than 1.35 [corresponding to the 90 per cent confidence level (dof= 29)] for 85 per cent of the spectra. We present here these parameter values in detail and consider their physical meaning.