Measuring the Spin of GRS 1915+105 With Relativistic Disk Reflection

GRS 1915+105 harbors one of the most massive known stellar black holes in the Galaxy. In 2007 May, we observed GRS 1915+105 for 117 ks in the low/hard state using Suzaku. We collected and analyzed the data with the Hard X-ray Detector/Positive Intrinsic Negative and X-ray Spectrometer cameras spanning the energy range from 2.3 to 55 keV. Fits to the spectra with simple models reveal strong disk reflection through an Fe K emission line and a Compton backscattering hump. We report constraints on the spin parameter of the black hole in GRS 1915 + 105 using relativistic disk reflection models. The model for the soft X-ray spectrum (i.e., < 10 keV) suggests = 0.56 +0.02 -0.02 and excludes zero spin at the 4 level of confidence. The model for the full broadband spectrum suggests that the spin may be higher, = 0.98 +0.01 -0.01 (1 confidence), and again excludes zero spin at the 2 level of confidence. We discuss these results in the context of other spin constraints and inner disk studies in GRS 1915 + 105.