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.

arXiv comment12 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 (25)

• ISGRI: the INTEGRAL Soft Gamma-Ray Imager
• Going with the flow: can the base of jets subsume the role of compact accretion disk coronae?
• The Two-Phase Pair Corona Model for Active Galactic Nuclei and X-ray Binaries: How to Obtain Exact Solutions
• Jets in neutron star X-ray binaries: a comparison with black holes
• The INTEGRAL/IBIS Scientific Data Analysis
• Hard X-ray Emission From Low Mass X-ray Binaries
• X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes
• Spectral Index and Quasi-Periodic Oscillation Frequency Correlation in Black Hole (BH) Sources: Observational Evidence of Two Phases and Phase Transition in BHs
• The radio luminosity of persistent X-ray binaries
• Detection of a Hard Tail in the X-ray Spectrum of the Z Source GX 349+2
• The broad band X-ray/hard X-ray spectra of accreting neutron stars
• On the Spectral Evolution of Cygnus X-2 along its Color-Color Diagram
• A peculiar spectral state transition of 4U1705--44: when an Atoll looks like a Z
• Simultaneous Measurements of X-Ray Luminosity and Kilohertz Quasi-Periodic Oscillations in Low-Mass X-Ray Binaries
• Comprehensive Analysis of RXTE Data from Cyg X-1: Spectral Index-Quasi-Periodic Oscillation Frequency-Luminosity Correlations
• Simultaneous BeppoSAX and RXTE observations of the X-ray burst sources GX 3+1 and Ser X-1
• Discovery of a 57-69 Hz quasi-periodic oscillation in GX13+1
• Simultaneous radio and X-ray observations of the low-mass X-ray binary GX 13+1
• The X-ray spectrum of the bursting atoll source 4U~1728-34 observed with INTEGRAL
• The transient hard X-ray tail of GX 17+2: new BeppoSAX results
• Disappearance of Hard X-ray Emission in the Last BeppoSAX Observation of the Z Source GX 349+2
• Resolving the Hard X-ray Emission of GX 5-1 with INTEGRAL
• The faint 2005 hard state outburst of Aquila X-1 seen by INTEGRAL and RXTE
• Simultaneous X-ray/optical observations of GX 9+9 (4U 1728-16)
• How To Distinguish Neutron Star and Black Hole X-ray Binaries? Spectral index and Quasi-Periodic Oscillation Frequency Correlation

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