1220027

Luminous accretion powered neutron stars generate majority of their X-ray emission in optically thick accretion disks and boundary/spreading layers. Numerous experiments in hard X-ray energy range show that nevertheless part of the energy release sometimes occurs in optically thin regime. This leads to generation of hard X-ray tail in spectra of luminous neutron stars. Among different physical mechanisms under discussion are Comptonization on thermal and non thermal population of electrons on bulk motion of matter in the accretion flow synchrotron emission of relativistic electrons. Scorpius X-1 is the brightest accretion powered neutron star on the sky therefore it is reasonable to validate all proposed theoretical models on data collected from this source. INTEGRAL data collected so far allowed us to obtain its spectrum at energies 20-300 keV and exclude some of the proposed models. INTEGRAL data have demonstrated that hard X-ray tail of Sco X-1 is highly variable with factor of more than 50. This means that highest statistical quality spectrum of Sco X-1 can be obtained not by averaging all of the available dataset but by collection of data during carefully selected periods of its enhanced activity. Thus the goal of our proposal is to monitor the behavior of hard X-ray emission of Sco X-1 with the aim to obtain the best statistical quality of hard X-ray part of its spectrum. Such increase of statistical quality will allow to further constrain physical models of generation of non thermal population of particles in the accretion flows. [truncated! Please see actual data for full text]