Scorpius X-1 (or Sco X-1 for short) is the brightest persistent source
in the X-ray sky. It was first extra-Solar source discovered more than
50 years ago and continues to be one of the best space laboratories. It
is a stellar binary, in which the compact object - a neutron star -
accretes matter from its companion (see Figure, top right). The mass
transfer rate in this binary is high, ensuring the very powerful luminosity
of the source - a few times 1038 erg/sec.
The vast majority of X-ray emission of the source is formed in an
optically thick accretion disk and optically thick layer on the neutron
star surface, where rapidly rotating matter of the accretion disk
settles onto it. It was established already early on that at even higher
energies, above 20-30 keV, Sco X-1 demonstrates an additional component,
continuing as a power law to energies above 100 keV. Its origin was not
clear for a long time. Among the hypotheses one suggestion is that the
hard X-ray emission originates as a result of scattering of some seed
photons on rapidly moving bulk flow near the neutron star.
The INTEGRAL observatory with its best available today sensitivity
above 100 keV, has devoted a large program to study this hard X-ray
emission of the prototype accreting neutron star binary Sco X-1. In total
approximately two months of observations were carefully analyzed. It is
shown that the hard X-ray emission of Sco X-1 does not rollover up to
energies above 200-300 keV (see Figure, top left) and can not be
produced by scattering of soft seed photons on bulk motion of matter
near the compact object. It is also shown that the presence of the hard
X-ray tail may be related to the existence of the inner part of the
optically thick disk; it is suggested that the hard X-ray emission
originates as a result of Compton up-scattering of soft X-rays from the
inner accretion disk by a population of non-thermal electrons, continuously
created in corona above the disk (see scheme on Figure, bottom right).
