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

Abstract

We studied and compared the long-term average hard X-ray (>20keV) 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 2-10keV range (H1750-440 and H1608-55). For all the sources of our sample, we analysed available public data and extracted average spectra from the IBIS/ISGRI detector on board INTEGRAL. 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 H1750-440 up to 150keV. 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 2.5-3keV. Six sources show a hard tail in their average spectrum: CygX-2 (Z), GX340+0 (Z), GX17+2 (Z), GX5-1 (Z), ScoX-1 (Z) and GX13+1 (Atoll). This is the first detection of a hard tail in the X-ray spectrum of the peculiar GX13+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 (40-100keV) 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.

12 pages, 7 figures, A&A in press; updated Fig 4 with an additional spectral state, typos corrected and added further comments for completeness

References in corpus (4)

• Comprehensive Analysis of RXTE Data from Cyg X-1: Spectral Index-Quasi-Periodic Oscillation Frequency-Luminosity Correlations
• The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
• Simultaneous X-ray/optical observations of GX 9+9 (4U 1728-16)
• The faint 2005 hard state outburst of Aquila X-1 seen by INTEGRAL and RXTE

Cited by in corpus (13)

• Modelling the behaviour of accretion flows in X-ray binaries
• IGR J16194-2810: a new symbiotic X-ray binary
• A new Comptonization model for low-magnetized accreting neutron stars in low mass X-ray binaries
• Linking Jet Emission, X-ray States and Hard X-ray Tails in the Neutron Star X-ray Binary GX 17+2
• BeppoSAX observation of 4U 1705-44: detection of hard X-ray emission in the soft state
• Polarised infrared emission from X-ray binary jets
• X-ray broad-band study of the symbiotic X-ray binary 4U 1954+31
• The first detection of Compton Reflection in the Low-Mass X-ray Binary 4U1705-44 with INTEGRAL and BeppoSAX
• Discovery of Red-Skewed K_alpha iron line in Cyg X-2 with Suzaku
• The first broad-band X-ray study of the Supergiant Fast X-ray Transient SAXJ1818.6-1703 in outburst
• Correlations between X-ray Spectral and Timing Characteristics in Cyg X-2
• The ultra-compact binary 4U1850-087 observed with INTEGRAL: hard X-ray emission from an X-ray burster
• The Spreading Layer of GX 9+9