http://arxiv.org/abs/1705.06638
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 {\it{NuSTAR}} and {\it{Suzaku}}. The same data were analysed before, and modelled by a lamppost containing hybrid electrons and located very close to the horizon, which 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 $\simeq$13–20 gravitational radii. On the other hand, we have also found a spectral solution with a lamppost at a large height and a disc that can extend to the innnermost stable circular orbit. Overall, we find the fitted truncation radius depends on the assumed continuum and geometry.
R. Basak, A. Zdziarski, M. Parker, et. al.
Fri, 19 May 17
31/62
Comments: submitted to MNRAS, comments are welcome