SPECTRAL VARIATIONS ALONG THE BRANCHES IN GX 5-1

We investigated spectral variations of the Z source GX 5-1 from the hi gh-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 tai l may be intrinsic to the source. The energy spectrum in the FB shows a significant deviation from the continuous spectra of models: i.e., t he sum of a blackbody and an unsaturated Comptonization spectrum, or a sum of a blackbody and multi-color disk model. The deviation can be a pproximated by a broad Gaussian profile (FWHM approximately 3 keV) cen tered at around 10 keV. The X-ray light curve in the FB is characteriz ed 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 centere d 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 possib le origins of the Gaussian component in the energy spectrum.