NuSTAR and Suzaku observations of the hard state in Cygnus X-1: locating the inner accretion disk

TL;DR

This study uses simultaneous NuSTAR and Suzaku observations to analyze the inner accretion disk of Cygnus X-1 in the hard state, revealing high black hole spin and a non-truncated disk close to the black hole.

Contribution

First simultaneous NuSTAR and Suzaku observations of Cygnus X-1 in the hard state, providing detailed reflection spectra and constraining the inner disk radius and black hole spin.

Findings

High black hole spin measured

Inner disk radius within 3 gravitational radii

Reflection spectrum consistent across states

Abstract

We present simultaneous Nuclear Spectroscopic Telescope Array (NuSTAR ) and Suzaku observations of the X-ray binary Cygnus X-1 in the hard state. This is the first time this state has been observed in Cyg X-1 with NuSTAR, which enables us to study the reflection and broad-band spectra in unprecedented detail. We confirm that the iron line cannot be fit with a combination of narrow lines and absorption features, and instead requires a relativistically blurred profile in combination with a narrow line and absorption from the companion wind. We use the reflection models of Garcia et al. (2014) to simultaneously measure the black hole spin, disk inner radius, and coronal height in a self-consistent manner. Detailed fits to the iron line profile indicate a high level of relativistic blurring, indicative of reflection from the inner accretion disk. We find a high spin, a small inner disk radius, and a low source height, and rule out truncation to greater than three gravitational radii at the 3{\sigma} confidence level. In addition, we find that the line profile has not changed greatly in the switch from soft to hard states, and that the differences are consistent with changes in the underlying reflection spectrum rather than the relativistic blurring. We find that the blurring parameters are consistent when fitting either just the iron line or the entire broad-band spectrum, which is well modelled with a Comptonized continuum plus reflection model.