MG11 
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Evolution of X-ray spectra of Neutron Star (NS) LMXBs: two blackbody-like components as a signatures of NS, accretion disk and transient hard X-ray tails as a Comptonization signature.

Transient hard X-ray tails extending above 100 keV in Neutron Star Low Mass X-Ray Binaries belonging to the Z class have been unambigously observed since the BeppoSAX and RXTE era. Their origin and the reason why they are transient is however unclear. Comptonization in a hybrid thermal plus non-thermal plasma related to the presence of relativistic jets has been claimed as a possible explanation. By mean of the analysis of broad-band data from the BeppoSAX archive on a sample of Z sources, we propose an alternative explanation: we suggest that bulk Comptonization processes mainly related to the accretion disk could produce these powerlaw-like excesses. We also investigate the timing properties of the sources in order to find which is the main parameter that drives the occurrence of the hard X-ray tails.