Using Suzaku observations of three neutron star low-mass X-ray binaries ( Ser X-1, 4U 1820-30, and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ("diskline''), allowing a measurement of the inner radius of the accretion disk and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5-16.5 km for a 1.4 M-circle dot neutron star. The inner disk radii that we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820-30 and GX 349+2, supporting the inner disk origin for kHz QPOs. In addition, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

/pdf/relativistic-iron-emission-lines-in-neutron-star-low-mass-x-1o2ei2wepp.pdf

Relativistic Iron Emission Lines in Neutron Star Low-Mass X-Ray Binaries as Probes of Neutron Star Radii

We report results of a BeppoSAX (0.1-200 keV) observation of the Z-type low-mass X-ray binary GX 17+2. The source was on the so-called horizontal and normal branches. Energy spectra were selected based on the source position in the X-ray hardness-intensity diagram. The continuum could be fairly well described by the sum of a ~0.6 keV blackbody, contributing ~10% of the observed 0.1-200 keV flux, and a Comptonized component, resulting from upscattering of ~1 keV seed photons by an electron cloud with temperature of ~3 keV and optical depth of ~10. Iron K line and edge were also present at energies of ~6.7 and ~8.5 keV, respectively. In the spectra of the horizontal branch, a hard tail was clearly detected at energies above ~30 keV. It could be fit by a power law of photon index ~2.7, contributing ~8% of the source flux. This component gradually faded as the source moved toward the normal branch, where it was no longer detectable. We discuss the possible origin of this component and the similarities with the spectra of atoll sources and black hole X-ray binaries.

The Discovery of a State-Dependent Hard Tail in the X-Ray Spectrum of the Luminous Z Source GX 17+2

We report on ten type I X-ray bursts originating from GX 17+2 in data obtained with the RXTE/PCA in 1996-2000. Three bursts were short in duration (∼10 s), whereas the others lasted for ∼6-25 min. All bursts showed spectral softening during their decay. There is no evidence for high-frequency (>100 Hz) oscillations at any phase of the bursts. We see no correlations of the burst properties with respect to the persistent X-ray spectral properties, suggesting that in GX 17+2 the properties of the bursts do not correlate with inferred mass accretion rate. The presence of short bursts in GX 17+2 (and similar bright X-ray sources) is not accounted for in the current X-ray bursts theories at the high mass accretion rates encountered in this source. We obtain satisfactory results if we model the burst emission with a black body, after subtraction of the persistent pre-burst emission. The two- component spectral model does not fit the total burst emission whenever there is a black-body component present in the persistent emission. We conclude that in those cases the black-body contribution from the persistent emission is also present during the burst. This implies that, contrary to previous suggestions, the burst emission does not arise from the same site as the persistent black-body emission. The black-body component of the persistent emission is consistent with being produced in an expanded boundary layer, as indicated by recent theoretical work. Five of the long bursts showed evidence of radius expansion of the neutron star photosphere (independent of the spectral analysis method used), presumably due to the burst luminosity reaching the Eddington value. When the burst luminosity is close to the Eddington value, slight deviations from pure black-body radiation are seen at energies below � 10 keV. Similar deviations have been seen during (long) X-ray bursts from other sources; they can not be explained by spectral hardening models. The total persistent flux just before and after the radius expansion bursts is inferred to be up to a factor of 2 higher than the net peak flux of the burst. If both the burst and persistent emission are radiated isotropically, this would imply that the persistent emission is up to a factor of 2 higher than the Eddington luminosity. This is unlikely and we suggest that the persistent luminosity is close to the Eddington luminosity and that the burst emission is (highly) anisotropic (ξ ∼ 2). Assuming that the net burst peak fluxes equal the Eddington limit, applying standard burst parameters (1.4 Mneutron star, cosmic composition, electron scattering opacity appropriate for high temperatures), and taking into account gravitational redshift and spectral hardening, we derive a distance to GX 17+2 of ∼8 kpc, with an uncertainty of up to ∼30%.

/pdf/x-ray-bursts-at-extreme-mass-accretion-rates-from-gx-17-2-2g26241fe4.pdf

X-ray bursts at extreme mass accretion rates from GX 17+2

We present solutions for the structure of the boundary layer where the accretion disk meets the neutron star, which is expected to be the dominant source of high-energy radiation in low-mass X-ray binaries which contain weakly magnetized accreting neutron stars. 
We find that the main portion of the boundary layer gas is hot (> ~10^8 K), low in density, radially and vertically extended, and optically thick to scattering but optically thin to absorption. It will produce large X-ray luminosity by Comptonization. Energy is transported inward by viscosity, concentrating the energy dissipation in the dense, optically thick zone close to the stellar surface. We explore the dependence of the boundary layer structure on the mass accretion rate, the rotation rate of the star, the alpha viscosity parameter and the viscosity prescription. 
Radiation pressure is the dominant source of pressure in the boundary layer; the flux is close to the Eddington limiting flux even for luminosities well below (~0.01 times) L(Edd). At luminosities near L(Edd), the boundary layer expands radially, and has a radial extent larger than one stellar radius. 
Based on the temperatures and optical depths which characterize the boundary layer, we expect that Comptonization will produce a power-law spectrum at low source luminosities. At high luminosities, a Planckian spectrum will be produced in the dense region where most of the energy is released, and modified by Comptonization as the radiation propagates outward.

Accretion Disk Boundary Layers Around Neutron Stars: X-ray Production in Low-Mass X-ray Binaries

The broad band x-ray/hard x-ray spectra of accreting neutron stars

Etude des proprietes spectrales d'un echantillon de six etoiles binaires RX de faible masse, a partir de donnees du satellite EXOSAT. On propose differents modeles du continuum de ces sources

/pdf/a-study-of-the-continuum-and-iron-k-line-emission-from-low-3n6antenrh.pdf

A study of the continuum and iron K line emission from low-mass X-ray binaries

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A study of the continuum and iron K line emission from low-mass X-ray binaries