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

• Published in: arXiv.org
• Main Authors: Basak, Rupal, Zdziarski, Andrzej A, Parker, Michael, Islam, Nazma
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 02.09.2017
• Subjects: Circular orbits; Continuum modeling; Deposition; Geometry; Orbital stability; Physics - High Energy Astrophysical Phenomena; Soft x rays; X ray binaries; X ray stars
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.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 NuSTAR and 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 spectral solution with a lamppost at a large height and a disc that can extend to the innnermost stable circular orbit, though somewhat statistically worse. Overall, we find the fitted truncation radius depends on the assumed continuum and geometry.