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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 472; no. 4; pp. 4220 - 4232
• Main Authors: Basak, Rupal, Zdziarski, Andrzej A., Parker, Michael, Islam, Nazma
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.12.2017
• Subjects: accretion; accretion discs; black hole physics; Circular orbits; Continuum modeling; Deposition; Geometry; Orbital stability; Soft x rays; stars: individual: Cyg X-1; X ray binaries; X ray stars; X- rays: binaries; X- rays: individual: Cyg X-1; black hole physics; accretion, accretion discs; X-rays: individual: Cyg X-1; stars: individual: Cyg X-1 – X-rays: binaries
• ISSN: 0035-8711; 1365-2966; 1365-2966
• DOI: 10.1093/mnras/stx2283

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 χ2 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.