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We present an analysis of the spectral and timing properties observed inX-rays from neutron star binary GX~3+1. We analyze all observations of thissource obtained with the RXTE and BeppoSAX satellites. We find that the X-raybroad-band energy spectra during these spectral transitions can be adequatelyreproduced by a composition of a low-temperature blackbody component, aComptonized component (COMPTB) and Gaussian component. We argue that theelectron temperature kT_e of the Compton cloud monotonically increases from 2.3keV to 4.5 keV, when GX~3+1 makes a spectral transition. Using a disk seedphoton normalization of COMPTB, which is proportional to mass accretion rate,we find that the photon power-law index Gamma is almost constant (Gamma=2.00+/-0.02) when mass accretion rate changes by factor 4. We interpret thisquasi-stability of the index Gamma and a particular form of the spectrum in theframework of a model in which the energy release in the transition layerlocated between the accretion disk and neutron star surface dominates that inthe disk. Moreover, this index stability effect now established for GX~3+1 waspreviously found in the atoll source 4U~1728-34 and suggested for a number ofother LMXB NS binaries (see Farinelli & Titarchuk). This intrinsic behavior ofneutron stars, in particular for atoll sources, is fundamentally different fromthat seen in black hole binary sources where the index monotonically increasesduring spectral transition from the low state to the high state and thenfinally saturates at high values of mass accretion rate.

GX~3+1: the stability of spectral index as a function of mass accretion rate