NuSTAR Spectroscopy of GRS 1915+105: Disk Reflection, Spin, and Connections to Jets
(Aug 21, 2013)Abstract
We report on the results of spectral fits made to a NuSTAR observation of the
black hole GRS 1915+105 in a "plateau" state. This state is of special interest
because it is similar to the "low/hard" state seen in other black holes,
especially in that compact, steady jets are launched in this phase. The 3-79
keV bandpass of NuSTAR, and its ability to obtain moderate-resolution spectra
free from distortions such as photon pile-up, are extremely well suited to
studies of disk reflection in X-ray binaries. In only 15 ks of net exposure, an
extraordinarily sensitive spectrum of GRS 1915+105 was measured across the full
bandpass. Ionized reflection from a disk around a rapidly-spinning black hole
is clearly required to fit the spectra; even hybrid Comptonization models
including ionized reflection from a disk around a Schwarzschild black hole
proved inadequate. A spin parameter of a = 0.98 +/- 0.01 (1-sigma statistical
error) is measured via the best-fit model; low spins are ruled out at a high
level of confidence. This result suggests that jets can be launched from a disk
extending to the innermost stable circular orbit. A very steep inner disk
emissivity profile is also measured, consistent with models of compact coronae
above Kerr black holes. These results support an emerging association between
the hard X-ray corona and the base of the relativistic jet.