Miller, J. M.3; Parker, M. L.4; Fuerst, F.12; Bachetti, M.6; Harrison, F. A.6; Barret, D.6; Boggs, S. E.7; Chakrabarty, D.13; Christensen, Finn Erland1; Craig, W. W.14; Fabian, A. C.4; Grefenstette, B. W.6; Hailey, C. J.15; King, A. L.3; Stern, D. K.12; Tomsick, J. A.7; Walton, D. J.12; Zhang, W. W.16
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 University of Michigan4 University of Cambridge5 California Institute of Technology6 Université de Toulouse7 University of California8 Massachusetts Institute of Technology9 Lawrence Livermore National Laboratory10 Columbia University11 NASA Goddard Space Flight Center12 California Institute of Technology13 Massachusetts Institute of Technology14 Lawrence Livermore National Laboratory15 Columbia University16 NASA Goddard Space Flight Center
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 (1σ 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.