NuSTAR Observations of the Black Hole GS 1354-645: Evidence of Rapid Black Hole Spin

Understand

We present the results of a NuSTAR study of the dynamically confirmed stellar-mass black hole GS 1354-645.

• The source was observed during its 2015 "hard" state outburst; we concentrate on spectra from two relatively bright phases.
• In the higher-flux observation, the broadband NuSTAR spectra reveal a clear, strong disk reflection spectrum, blurred by a degree that requires a black hole spin of a = cJ/GM^2 > 0.98 (1 sigma statistical limits only).
• The fits also require a high inclination: theta = 75(2) degrees.