EDP Sciences
Free access
Volume 443, Number 2, November IV 2005
Page(s) 599 - 608
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20041647

A&A 443, 599-608 (2005)
DOI: 10.1051/0004-6361:20041647

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

A. Paizis1, 2, K. Ebisawa3, 1, T. Tikkanen4, J. Rodriguez5, 1, J. Chenevez6, E. Kuulkers7, O. Vilhu4 and T. J.-L. Courvoisier1, 8

1  INTEGRAL 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 )

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.

Key words: X-rays: binaries -- binaries (including multiple): close -- stars: neutron -- stars: individual: GX 5-1

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© ESO 2005

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