Average hard X-ray emission from NS LMXBs: observational evidence of different spectral states in NS LMXBs

¹ INAF-IASF, Sezione di Milano, via Bassini 15, 20133 Milano, Italy e-mail: ada@iasf-milano.inaf.it
² Dipartimento di Fisica, Università di Ferrara, via Saragat 1, 44100 Ferrara, Italy
³ NASA Goddard Space Flight Center, Exploration of the Universe Division, Greenbelt, MD 20771, USA
⁴ George Mason University/Center for Earth Observing and Space Research, Fairfax, VA 22030; and US Naval Research Laboratory, Code 7655, Washington, DC 20375-5352, USA
⁵ Science Data Centre, Chemin d'Ecogia 16, 1290 Versoix, Switzerland
⁶ Observatoire de Genève, 51 chemin des Mailletes, 1290 Sauverny, Switzerland
⁷ INAF-IASF, Sezione di Roma, via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁸ Joint Center for Astrophysics, Department of Physics, University of Maryland, Baltimore County, MD 21250, USA
⁹ APC/UMR 7164, 11 Place M. Berthelot, 75231 Paris, France
¹⁰ CEA, Centre de Saclay, DSM/DAPNIA/SAp, 91191 Gif-sur-Yvette Cedex, France
¹¹ ISOC, ESA/ESAC, Urb. Villafranca del Castillo, PO Box 50727, 28080 Madrid, Spain
¹² Observatory, PO Box 14, Tähtitorninmäki, 00014 University of Helsinki, Finland

Received: 9 June 2006
Accepted: 10 August 2006

Abstract

Aims.We studied and compared the long-term average hard X-ray (>20 keV) spectra of a sample of twelve bright low-mass X-ray binaries hosting a neutron star (NS). Our sample comprises the six well studied Galactic Z sources and six Atoll sources, four of which are bright (“GX”) bulge sources while two are weaker ones in the 2–10 keV range (H 1750–440 and H 1608–55).

Methods.For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL.

Results.We can describe all the spectral states in terms of the bulk motion Comptonisation scenario. We find evidence that bulk motion is always present, its strength is related to the accretion rate and it is suppressed only in the presence of high local luminosity. The two low-dim Atoll source spectra are dominated by photons up-scattered presumably due to dynamical and thermal Comptonisation in an optically thin, hot plasma. For the first time, we extend the detection of H 1750–440 up to 150 keV. The Z and bright “GX” Atoll source spectra are very similar and are dominated by Comptonised blackbody radiation of seed photons, presumably coming from the accretion disc and NS surface, in an optically thick cloud with plasma temperature in the range of 2.5–3 keV. Six sources show a hard tail in their average spectrum: Cyg X-2 (Z), GX 340+0 (Z), GX 17+2 (Z), GX 5–1 (Z), Sco X–1 (Z) and GX 13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX 13+1. Using radio data from the literature we find, in all Z sources and bright “GX” Atolls, a systematic positive correlation between the X-ray hard tail (40–100 keV) and the radio luminosity. This suggests that hard tails and energetic electrons causing the radio emission may have the same origin, most likely the Compton cloud located inside the NS magnetosphere.