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The aim of this work is to investigate in a physical and quantitative way thespectral evolution of bright Neutron Star Low-Mass X-ray Binaries (NS LMXBs),with special regard to the transient hard X-ray tails. We analyzed INTEGRALdata for five sources (GX 5-1, GX 349+2, GX 13+1, GX 3+1, GX 9+1) and builtbroad-band X-ray spectra from JEM-X1 and IBIS/ISGRI data. For each source,X-ray spectra from different states were fitted with the recently proposedmodel compTB. The spectra have been fit with a two-compTB model. In all casesthe first compTB describes the dominant part of the spectrum that we interpretas thermal Comptonization of soft seed photons (< 1 keV), likely from theaccretion disk, by a 3-5 keV corona. In all cases, this component does notevolve much in terms of Comptonization efficiency, with the system convergingto thermal equilibrium for increasing accretion rate. The second compTB variesmore dramatically spanning from bulk plus thermal Comptonization of blackbodyseed photons to the blackbody emission alone. These seed photons (R < 12 km,kT_s > 1 keV), likely from the neutron star and the innermost part of thesystem, the Transition Layer, are Comptonized by matter in a converging flow.The presence and nature of this second compTB component (be it a pure blackbodyor Comptonized) are related to the inner local accretion rate which caninfluence the transient behaviour of the hard tail: high values of accretionrates correspond to an efficient Bulk Comptonization process (bulk parameterdelta > 0) while even higher values of accretion rates suppress theComptonization, resulting in simple blackbody emission (delta=0).

Spectral evolution of bright NS LMXBs with INTEGRAL: an application of the thermal plus bulk Comptonization model