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- Basak, Rupal, et al.
(författare)
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Analysis of NuSTAR and Suzaku observations of Cyg X-1 in the hard state : evidence for a truncated disc geometry
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 472:4, s. 4220-4232
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Tidskriftsartikel (refereegranskat)abstract
- 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 chi(2) and has a geometry with a hot inner accretion flow and a disc truncated at similar or equal to 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.
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