2005A&A...443..599P

We present the study of one year of INTEGRAL data on the neutron star low mass X-ray binary GX 5-1. Thanks to the excellent angular resolution and sensitivity of INTEGRAL, we are able to obtain a high quality spectrum of GX 5-1 from ∼5keV to ∼100keV, for the first time without contamination from the nearby black hole candidate GRS 1758-258 above 20keV. During our observations, GX 5-1 was mostly found in the horizontal and normal branch of its hardness intensity diagram. A clear hard X-ray emission is observed above ∼30keV which exceeds the exponential cut-off spectrum expected from lower energies. This spectral flattening may have the same origin of the hard components observed in other Z sources as it shares the property of being characteristic to the horizontal branch. The hard excess is explained by introducing Compton up-scattering of soft photons from the neutron star surface due to a thin hot plasma expected in the boundary layer. The spectral changes of GX 5-1 downward along the ``Z'' pattern in the hardness intensity diagram can be well described in terms of monotonical decrease of the neutron star surface temperature. This may be a consequence of the gradual expansion of the boundary layer as the mass accretion rate increases.