NuSTAR OBSERVATIONS OF THE BLACK HOLE GS 1354–645: EVIDENCE OF RAPID BLACK HOLE SPIN

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/GM2 ≥ 0.98 (1σ statistical limits only). The fits also require a high inclination: θ ≃ 75(2). 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. Subject headings: accretion disks – black hole physics – X-rays: binaries