Resolving the hard X-ray emission of GX 5-1 with INTEGRAL

¹ Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland e-mail: Ada.Paizis@obs.unige.ch
² INAF-IASF, Sezione di Milano, via Bassini 15, 20133 Milano, Italy
³ NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA
⁴ Observatory, PO Box 14, Tähtitorninmäki, 00014 University of Helsinki, Finland
⁵ CNRS, FRE 2591, CE Saclay DSM/DAPNIA/SAp, 91191 Gif-sur-Yvette Cedex, France
⁶ Danish Space Research Institute, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁷ Research and Scientific Support Department of ESA, ESTEC, PO Box 299, 2200 AG Noordwijk, The Netherlands
⁸ Observatoire de Genève, 51 chemin des Mailletes, 1290 Sauverny, Switzerland

Received: 12 July 2004
Accepted: 21 July 2005

Abstract

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 ~5 keV to ~100 keV, for the first time without contamination from the nearby black hole candidate GRS 1758-258 above 20 keV. 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 ~30 keV 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.