  GRS 1915+105 harbors one of the most massive known stellar black holes in the
Galaxy. In May 2007, we observed GRS 1915+105 for 117 ksec in the low/hard
state using Suzaku. We collected and analyzed the data with the HXD/PIN and XIS
cameras spanning the energy range from 2.3-55 keV. Fits to the spectra with
simple models reveal strong disk reflection through an Fe K emission line and a
Compton back-scattering 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 a/M = 0.56(2) and
excludes zero spin at the 4 sigma level of confidence. The model for the full
broadband spectrum suggests that the spin may be higher, a/M = 0.98(1) (1 sigma
confidence), and again excludes zero spin at the 2 sigma level of confidence.
We discuss these results in the context of other spin constraints and inner
disk studies in GRS 1915+105.
