The Soft State of Cygnus X-1 Observed with NuSTAR: A Variable Corona and a Stable Inner Disk

Understand

We present a multi-epoch hard X-ray analysis of Cygnus X-1 in its soft state based on four observations with NuSTAR.

• Despite the basic similarity of the observed spectra, there is clear spectral variability between epochs.
• To investigate this variability, we construct a model incorporating both the standard disk-corona continuum and relativistic reflection from the accretion disk, based on prior NuSTAR work on Cygnus X-1, and apply this model to each epoch independently.
• We find excellent consistency for the black hole spin, and the iron abundance of the accretion disk, which are expected to remain constant on observational timescales.