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

TL;DR

This study uses NuSTAR X-ray observations to measure a very high spin parameter (>0.98) of the stellar-mass black hole GS 1354-645 during an outburst, revealing insights into accretion disk geometry.

Contribution

First measurement of near-maximal black hole spin in GS 1354-645 using broadband X-ray spectra during an outburst.

Findings

Black hole spin a > 0.98 with 1 sigma confidence

High inclination angle of 75 degrees

Evidence of disk truncation at lower flux states

Abstract

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. Strong "dips" are sometimes observed in the X-ray light curves of sources viewed at such an angle; these are absent, perhaps indicating that dips correspond to flared disk structures that only manifest at higher accretion rates. In the lower-flux observation, there is evidence of radial truncation of the thin accretion disk. We discuss these results in the context of spin in stellar-mass black holes, and inner accretion flow geometries at moderate accretion rates.