Abstract
Context. Cygnus X-1 is a black hole X-ray binary system in which the black hole captures and accretes gas from the strong stellar wind emitted by its supergiant O9.7 companion star. The irradiation of the supergiant star essentially determines the flow properties of the stellar wind and the X-ray luminosity from the system. The results of three-dimensional hydrodynamical simulations of wind-fed X-ray binary systems reported in recent work reveal that the ionizing feedback of the X-ray irradiation leads to the existence of two stable states with either a soft or a hard spectrum. Aims. We discuss the observed radiation of Cygnus X-1 in the soft and hard state in the context of mass flow in the corona and disk, as predicted by the recent application of a condensation model. Methods. The rates of gas condensation from the corona to the disk for Cygnus X-1 are determined, and the spectra of the hard and soft radiation are computed. The theoretical results are compared with the MAXI observations of Cygnus X-1 from 2009 to 2018. In particular, we evaluate the hardness-intensity diagrams (HIDs) for its ten episodes of soft and hard states which show that Cygnus X-1 is distinct in its spectral changes as compared to those found in the HIDs of low-mass X-ray binaries. Results. The theoretically derived values of photon counts and hardness are in approximate agreement with the observed data in the HID. However, the scatter in the diagram is not reproduced. Improved agreement could result from variations in the viscosity associated with clumping in the stellar wind and corresponding changes of the magnetic fields in the disk. The observed dipping events in the hard state may also contribute to the scatter and to a harder spectrum than predicted by the model.
Highlights
Cygnus X-1 is one of the most frequently observed high-mass X-ray binaries (HMXB) having been studied with instruments on ground-based and space-based telescopes
The condensation rates are calculated according to the formulae given in the investigation by Liu et al (2011) and Qiao & Liu (2012), including the irradiation, which is important for Cyg X1
Upon comparison of the results of the computations with the observations for the years 2009–2018 in Fig. 5, we find that the condensation process can yield photon count rates and hardness in the soft state as observed for Cyg X-1 for the assumed mass supply rates, provided that the viscosity parameter is in the range of 0.1−0.2
Summary
Cygnus X-1 is one of the most frequently observed high-mass X-ray binaries (HMXB) having been studied with instruments on ground-based and space-based telescopes. A fraction of the mass lost from the O/B star is likely accreted in the form of a coronal flow possibly forming an inner accretion disk, which results in either a hard or soft X-ray spectrum. The observations of Cygnus X-1 (hereafter Cyg X-1) over several decades record the long-lasting hard and soft spectral states. The spectra in the hard and soft states and the change between spectral states have been observationally studied on the one hand, while the formation and structure of the wind of the O/B star, including the influence of irradiation have been theoretically studied on the other. Studies of the similarities and differences in comparison with transient X-ray binaries (e.g., GX 339−4), the short-time and long-time variability, and the presence of radio emission and γ-ray emission have been undertaken
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