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

## TL;DR

This study analyzes simultaneous NuSTAR and Suzaku observations of Cyg X-1 in the hard state, providing evidence for a truncated accretion disc geometry with a hot inner flow, challenging previous models of the accretion structure.

## Contribution

It offers a re-analysis of the continuum model, highlighting the importance of physical self-consistency and demonstrating that the disc is likely truncated at 13-20 gravitational radii.

## Key findings

- Evidence for a truncated disc at 13-20 gravitational radii.
- Presence of a soft thermal-Comptonization component in the spectrum.
- Spectral solutions vary depending on the assumed continuum and geometry.

## 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, 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.