We report on Rossi X-Ray T iming Explorer observations of four type I X-ray bursters, namely, 1E 1724 [3045, GS 1826[238, SLX 1735[269, and KS 1731 The Ðrst three were in a low state, with 1È200 keV X-ray luminosities in the range Eddington luminosity for a neutron D0.05È0.1L Edd (L Edd : star, ergs s~1), whereas KS 1731[260 was in a high state, with luminosity L Edd \ 2.5 ] 1038 D0.35L Edd . The low-state sources have very similar power spectra, displaying high-frequency noise up to D200 Hz. For KS 1731[260, its power spectrum is dominated by noise at frequenciesHz ; in addition a [20 quasi-periodic oscillation at 1200 Hz is detected in a segment of the observation. The 1È200 keV spectra of the low-state sources are all consistent with resulting from thermal Comptonization with an electron temperature around 25È30 keV. For KS 1731[260, the spectrum is also dominated by thermal (kT e ) Comptonization, but with a much lower keV and no signiÐcant hard X-ray emission. With the kT e D 3 exception of GS 1826[238, they each have an underlying soft component, carrying at most D25% of the total 1È200 keV luminosity. For all sources, we have detected an iron Ka line at 6.4 keV (although it is weak and marginal in 1E 1724 [3045). A reÑection component is present in the spectra of GS 1826[238 and SLX 1735[269, and for both we Ðnd that the reÑecting medium subtends only a small solid angle ()/2n D 0.15, 0.28). The origin of the line and the reÑection component is most likely to be irradiation of the accretion disk by the X-ray source. We suggest a model in which the region of main energy release, where hard X-rays are produced, would be an optically thin boundary layer merged with an advection-dominated accretion Ñow (ADAF) and would be responsible for the rapid variability observed. The soft component observed probably represents the unscattered emission from an optically thick accretion disk of variable inner radius. When the accretion rate increases, the inner disk radius shrinks and the strength of the reÑected component and associated iron line increase. At the same time, the Comptonization region cools o † in response to an increased cooling Ñux from the accretion disk and from the reprocessed/reÑected component, thus leading progressively to a quenching of the hard X-ray emission. If low-state neutron stars (NSs) accrete via ADAFs, the observation of X-ray bursts, indicating that all the accreting matter actually accumulates onto the NS surface, argues against the existence of strong winds from such accretion Ñows. Finally, we discuss two criteria recently proposed to distinguish between nonquiescent black holes (BHs) and NSs that are not contradicted by existing observations. The Ðrst one states that, when thermal Comptonization is responsible for the hard X-ray emission, only BHs have larger than D50 keV. However, this criterion is weakened by the fact that there are NSs diskT e playing nonattenuated power laws extending up to at least 200 keV, possibly implying nonthermal Comptonization or thermal Comptoni...

We study in a systematic way the quality factor of the lower and upper kilohertz quasi‐periodic oscillations (kHz QPOs) in a sample of low‐luminosity neutron star X‐ray binaries, showing both QPOs varying over a wide frequency range. The sample includes 4U 1636−536, 4U 1608−522, 4U 1735−44, 4U 1728−34, 4U 1820−303 and 4U 0614+09. We find that all sources except 4U 0614+09 show evidence of a drop in the quality factor of their lower kHz QPOs at high frequency. For 4U 0614+09 only the rising part of the quality factor versus frequency curve has been sampled so far. At the same time, in all sources but 4U 1728−34, the quality factor of the upper kHz QPO increases all the way to the highest detectable frequencies. We show that the high‐frequency behaviours of both the lower and the upper kHz QPO quality factors are consistent with what is expected if the drop is produced by the approach of an active oscillating region to the innermost stable circular orbit: the existence of which is a key feature of general relativity in the strong field regime. Within this interpretation, our results imply gravitational masses around 2 M⊙ for the neutron stars in those systems.