  Using the data from the Proportional Counter Array (PCA) and the High-Energy
X-ray Timing Experiment (HEXTE) on board it Rossi X-Ray Timing Explorer for Z
source GX 17+2, we investigate the evolution of its PCA spectra and HEXTE
spectra along a "Z" track on its hardness-intensity diagram. A hard X-ray tail
is detected in the HEXTE spectra. The detected hard X-ray tails are
discontinuously scattered throughout the "Z" track. The found hard tail hardens
from the horizontal branch, through the normal branch, to the flaring branch in
principle and it contributes ~(20-50)% of the total flux in 20-200 keV. Our
joint fitting results of the PCA+HEXTE spectra in 3-200 keV show that the
portion of Comptonization in the bulk-motion Comptonization (BMC) model
accounts for the hard X-ray tail, which indicates that the BMC process could be
responsible for the detected hard tail. The temperature of the seed photons for
BMC is ~2.7 keV, implying that these seed photons might be emitted from the
surface of the neutron star (NS) or the boundary layer between the NS and the
disk and, therefore, this process could take place around the NS or in the
boundary layer.
