0720011

Over last years it was firmly established that luminous accreting neutron stars demonstrate high energy tails in their spectra which while is very minor in terms of energy budget of a source can tell us a lot about physical processes occuring in the innermost regions of the accretion flow. In particular one can anticipate appearance of the high energy tail due to presence of hot optically thin corona above optically thick main emitting regions of the flow which might upscatter soft photons on its thermal or non thermal population of electrons. The main observational apperance which might help us to determine the origin of the tail is its behavior at high energies. The presence of high energy cutoff in this component will allow to distinguish between thermal and non thermal population of electrons and also will allow to put constrains on possible influence of so called bulk motion comptonization on formation of the high energy component. INTEGRAL observatory is the only facility which can provide constrains on the physics of the high energy component of luminous accreting neutron stars. It has abilities to study photon spectra at energies more than 100-200 keV with unprecedented accuracy. The limits obtained in the proposed study will not be overcome by any existing and planning instruments for a long time. We suggest to perform 4 Msec-long observation of Sco X-1 split over two AOs more than tripling the existing exposure. Among all possible galactic NS systems in the appropriate spectral state Sco X-1 is by far the brightest one (including high energy tail). This makes this object a must do target for the proposed study. [truncated! Please see actual data for full text]