Spectral Variations along the Branches in GX 5-1

We investigated spectral variations of the Z source GX 5-1 from the high-intensity end of the normal branch (NB) through the flaring branch (FB). We detected a clear hard-energy tail, which can be approximated by a power-law with a photon index of 1.8 and its intensity decreased by an order of magnitude from the NB to the FB. Therefore the hard tail may be intrinsic to the source. The energy spectrum in the FB shows a significant deviation from the continuous spectra of models: i.e., the sum of a blackbody and an unsaturated Comptonization spectrum, or a sum of a blackbody and multi-color disk model. The deviation can be approximated by a broad Gaussian profile (FWHM ~ 3 keV) centered at around 10 keV. The X-ray light curve in the FB is characterized by a dipping behavior on time scales of a few tens of seconds. The cross-correlation coefficient of the time variations in various energy bands against those in the 1.7--4.0 keV shows a peak structure centered at around 11 keV. This means that the Gaussian-like structure is not an artifact due to a particular selection of the continuum model. We discuss the origin of the hard-energy tail and consider several possible origins of the Gaussian component in the energy spectrum.