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

Aims. The aim of this work is to investigate in a physical and quantitative way the spectral evolution of bright Neutron Star Low-Mass X-ray Binaries (NS LMXBs), with special regard to the transient hard X-ray tails. Methods. We analyzed INTEGRALdata for five sources (GX 5‐1, GX 349 +2, GX 13+1, GX 3+1, GX 9+1) and built broad-band X‐ray spectra from JEM‐X1 and IBIS/ISGRI data. For each source, X-ray spectra from different states were fitted with the recent ly proposed model compTB. Results. The spectra have been fit with a two- compTB model. In all cases the first compTB describes the dominant part of the spectrum that we interpret as thermal Comptonization of soft seed photons ( 1 keV), likely from the neutron star and the innermost part of the system, the Transition Layer, are Comptonized by matter in a converging flow. The presence and nature of this second compTB component (be it a pure blackbody or Comptonized) are related to the inner local accretion rate which can influence the transient behaviour of the hard tail: high values of accretion rates correspond to an efficient Bulk Com ptonization process (bulk parameter � 6 0) while even higher values of accretion rates suppress the Comptonization, resulting in simple blackbody emission (� = 0). Conclusions. The spectral evolution of the sources has been successfully studied in terms of thermal and Bulk Comptonization efficien cy, in relation to the physical conditions in the Transition Layer.