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.
Original language | English (US) |
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Article number | A66 |
Journal | Astronomy and Astrophysics |
Volume | 637 |
DOIs | |
State | Published - May 1 2020 |
Funding
Acknowledgements. This research has made use of the MAXI data provided by RIKEN, JAXA and the MAXI team. In particular we thank Dr. Mutsumi Sugizaki for providing new data. Financial support for this work is provided by the National Program on Key Research and Development Project (Grant No. 2016YFA0400804), the gravitational wave pilot B (Grant No. XDB23040100), and the National Natural Science Foundation of China (Grant No. 11673026,11773037). We thank Friedrich Meyer and Weimin Yuan for discussions.
Keywords
- Binaries: close
- Outflows
- Stars: individual: Cygnus X-1
- Stars: winds
- X-rays: binaries
ASJC Scopus subject areas
- Astronomy and Astrophysics
- Space and Planetary Science