Generalized spectral model for 1-100 keV X-ray emission from Cygnus X-3 based on EXOSAT data

Abstract

The X-ray spectrum of the highly variable X-ray source, Cyg X-3, has so far defied a consistent explanation based on simple emission models. We have extracted two of the best data sets from the EXOSAT archives and performed a detailed spectral analysis for its 'high' and 'low' states. The analysis of the less frequently occurring 'low' state is presented for the first time for the EXOSAT data. Combining data from the medium-energy argon and xenon detectors and the gas scintillation proportional counter, with a better energy resolution, and carrying out a simultaneous fit, we find that the X-ray continuum in both the 'high' and 'low' state can be explained as a sum of a blackbody emission and emission from a Comptonized plasma cloud with a common absorption. The Comptonization model is sufficient as well as preferable to many other models, in explaining the observed X-ray emission up to 100 keV. In addition, we find an emission-line feature due to ionized iron (Fe XX-Fe XXVI) and absorption features due to cold iron (Fe I) as well as highly ionized iron (Fe XXV-Fe XXXVI). The presence of absorption due to Fe I has been shown for the first time here. This is the simplest and the most generalized spectral model for the 1-100 keV X-ray emission from Cyg X-3, to date. We find that the blackbody temperature derived in the 'high ' state (1.47 keV) is much lower than that derived for the 'low' state (2.40 keV) and is associated with an increase in the blackbodly radius in the 'high' state. The ratio of blackbody flux to the total flux is approximately 0.61 in the 'high' state and ~0.44 in the 'low' state. The Fe line energy is significantly higher in the 'high' state (~6.95 keV) compared to the 'low' state (~6.56 keV). The Comptonization parameter changes from 2 to ~15 in going from the 'high' to the 'low' state implying a highly saturated Comptonization in the 'low' state. The Comptonized region has high electron temperature and low opacity in the 'high' state and vice versa in the 'low' state. The orbital light curve is mostly explained by variations in the intensities of the continuum components. We discuss the likely origin of different emission regions, continuum and line, and interpret them in terms of an accretion disk corona.