• DOI:10.1093/mnras/stx2283
• Corpus ID: 39195066

Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry

@article{Basak2017AnalysisON,
  title={Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state: evidence for a truncated disc geometry},
  author={Rupal Basak and Andrzej A. Zdziarski and Michael L. Parker and Nazma Islam},
  journal={Monthly Notices of the Royal Astronomical Society},
  year={2017},
  volume={472},
  pages={4220-4232}
}

• R. Basak, A. Zdziarski, +1 author N. Islam
• Published 18 May 2017
• Physics
• Monthly Notices of the Royal Astronomical Society

The geometry of the accretion flow in black hole X-ray binaries in the hard state, in particular the position of the disc inner edge, has been a subject of intense debate in recent years. We address this issue by performing a spectral study of simultaneous observations of Cyg X-1 in the hard state by NuSTAR and Suzaku. The same data were analysed before, and modelled by a lamppost containing hybrid electrons and located very close to the horizon, whose emission was incident on a surrounding disc extending almost to the innermost stable circular orbit. We re-analyse the incident continuum model and show that it suffers from the lack of physical self-consistency. Still, the good fit to the data provided by this model indicates that the real continuum has a similar shape. We find it features a strong soft X-ray excess below a few keV, which we model as a soft thermal-Comptonization component, in addition to the main hard thermal-Compton component. This continuum model with reflection of both components yields the overall lowest χ^2 and has a geometry with a hot inner accretion flow and a disc truncated at ≃13–20 gravitational radii. On the other hand, we have also found spectral solution with a lamppost at a large height and a disc that can extend to the innermost stable circular orbit, though somewhat statistically worse. Overall, we find that the fitted truncation radius depends on the assumed continuum and geometry.