Neutron Star Low-mass X-ray Binaries Research Articles

On the kHz QPO frequency correlations in bright neutron star X-ray binaries

We re-examine the correlation between the frequencies of upper and lower kHz quasi-periodic oscillations (QPO) in bright neutron-star low-mass X-ray binaries. By including the kHz QPO frequencies of the X-ray binary Cir X-1 and two accreting millisecond pulsars in our sample, we show that the full sample does not support the class of theoretical models based on a single resonance, while models based on relativistic precession or Alfven waves describe the data better. Moreover, we show that the fact that all sources follow roughly the same correlation over a finite frequency range creates a correlation between the linear parameters of the fits to any sub-sample.

Rossi X‐Ray Timing ExplorerObservations of the First Transient Z Source XTE J1701−462: Shedding New Light on Mass Accretion in Luminous Neutron Star X‐Ray Binaries

We report on the first 10 weeks of RXTE observations of the X-ray transient XTE J1701-462 and conclude that it had all the characteristics of the neutron star Z sources, i.e., the brightest persistent neutron star low-mass X-ray binaries. These include the typical Z-shaped tracks traced out in X-ray color diagrams and the variability components detected in the power spectra, such as kHz QPOs and normal and horizontal branch oscillations. XTE J1701-462 is the first transient Z source and provides unique insights into mass accretion rate () and luminosity dependencies in neutron star X-ray binaries. As its overall luminosity decreased, we observed a switch between two types of Z source behavior, with the branches of the Z track changing their shape and/or orientation. We interpret this as an extreme case of the more moderate long-term changes seen in the persistent Z sources and suggest that they result from changes in . We also suggest that the Cyg-like Z sources (Cyg X-2, GX 5-1, and GX 340+0) are substantially more luminous (>50%) than the Sco-like Z sources (Sco X-1, GX 17+2, and GX 349+2). Adopting a possible explanation for the behavior of kHz QPOs, which involves a prompt as well as a filtered response to changes in , we further propose that changes in can explain both movement along the Z track and changes in the shape of the Z track. We discuss some consequences of this and consider the possibility that the branches of the Z will smoothly evolve into the branches observed in X-ray color diagrams of the less luminous atoll sources, although not in a way that was previously suggested.

An Accreting Millisecond Pulsar with Black Hole–like X‐Ray Variability: IGR J00291+5934

IGR J00291+5934 is one of the seven accreting millisecond pulsars (AMPs) discovered so far. We report on the aperiodic timing and color analysis of its X-ray flux, using all the RXTE observations of the 2004 outburst. Flat-top noise and two harmonically related quasi-periodic oscillations, all of them at very low frequencies (0.01-0.1 Hz), were present in the power spectra during most of the outburst as well as a very high fractional variability (~50%). These properties are atypical not only for AMPs but also for neutron star low-mass X-ray binaries (LMXBs) in general. There are instead some remarkable similarities with the variability observed in black hole systems, reinforcing the connections between these two types of LMXB, as well as some interesting differences. We note finally that the results of this paper are difficult to reconcile with interpretations where any break frequency of power density spectra scales inversely with the mass of the central object at an accuracy sufficient to distinguish between the masses of neutron stars and black holes in LMXBs.

Supporting evidence for the signature of the innermost stable circular orbit in Rossi X-ray data from 4U 1636-536

Analysis of archival Rossi X-ray Timing Explorer (RXTE) data on neutron star low-mass X-ray binaries has shown that for several sources the quality factor Q of the lower kilohertz quasi- periodic oscillations (QPO) drops sharply beyond a certain frequency. This is one possible signature of the approach to the general relativistic innermost stable circular orbit (ISCO), but the implications of such an interpretation for strong gravity and dense matter are important enough that it is essential to explore alternate explanations. In this spirit, Mendez has recently proposed that Q depends fundamentally on mass accretion rate (as measured by spectral hardness) rather than the frequency of the QPO. Specifically, he has suggested that analysis of multiple sources shows trends in Q similar to those previously reported in individual sources, and he surmises that the ISCO therefore does not play a role in the observed sharp drop in Q in any source. We test this hypothesis for 4U 1636−536 by measuring precisely spectral colours simultaneously with the lower QPO frequency and Q after correction for the frequency drift, over a data set spanning eight years of RXTE observations. We find that in this source there is no correlation between Q and spectral hardness. In particular, no apparent changes in hardness are observed when Q reaches its maximum before dropping off. We perform a similar analysis on 4U 1608−522; another source showing a sharp drop in the quality factor of its lower kHz QPO. We find that for this source, positive and negative correlations are observed between spectral hardness, frequency and Q. Consequently, if we are to search for a common explanation for the sharp drop in the quality factor seen in both sources, the spectral hardness is not a good candidate for the independent variable whereas the frequency remains. Therefore, we conclude that the ISCO explanation is viable for 4U 1636−536, and thus possibly for others.

Study on the correlations between the twin kilohertz quasi-periodic oscillations in low-mass X-ray binaries

The recently updated data of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in the neutron star low-mass X-ray binaries are analyzed. The power-law fitting ν 1 = a( ν 2/1000) b and linear fitting ν 2 = Aν 1 + B are applied, individually, to the data points of four Z sources (GX 17+2, GX 340+0, GX 5−1 and Sco X−1) and four Atoll sources (4U 0614+09, 4U 1608−52, 4U 1636−53 and 4U 1728−34). The χ 2-tests show that the power-law correlation and linear correlation both can fit data well. Moreover, the comparisons between the data and the theoretical models for kHz QPOs are discussed.

A population synthesis study on the faint X-ray sources in the Galactic center region

Context.Recent Chandra observations of the Galactic center region (GCR) have uncovered a population of faint discrete X-ray sources. A few theoretical works have already investigated the nature of these sources.Aims.We examine the contributions and luminosity functions of various kinds of candidate objects that have been proposed either by previous authors or by ourselves.Methods.We conduct a population synthesis calculation based on the Hurley et al. rapid binary evolution code. Several candidate models, i.e. wind-accreting neutron stars, intermediate polars, low mass X-ray binaries, young pulsars, and massive stars with strong winds, are incorporated into our calculation. We also take the geometric effect of the accretion disk into account for Roche lobe overflow X-ray binaries.Results.Our results show that neutron star low-mass X-ray binaries contribute significantly to the observed sources. We also point out that wind-accreting neutron stars contribute negligible amount to these sources due to the propeller effect, and the intermediate polars play a minor role in accounting for the faint X-ray sources in both the Wang et al. and Muno et al. survey. It should be mentioned that the majority of the sources in the survey field of Wang et al. are still beyond what we can expect to find.

Discovery of the Millisecond X‐Ray Pulsar HETE J1900.1−2455

We report the discovery of millisecond pulsations from the low-mass X-ray binary HETE J1900.1-2455 which was discovered by the detection of a type I X-ray burst by the High Energy Transient Explorer 2 (HETE-2). The neutron star emits coherent pulsations at 377.3 Hz and is in an 83.3 minute circular orbit with a companion with a mass greater than 0.016 solar masses and likely less than 0.07 solar masses. The companion star's Roche lobe could be filled by a brown dwarf with no need for heating or non-standard evolution. During one interval with an unusually high X-ray flux, the source produced quasiperiodic oscillations with a single peak at 883 Hz and on subsequent days, the pulsations were suppressed. We consider the distribution of spin versus orbital period in neutron star low-mass X-ray binaries.

The correlations between the twin kHz quasi-periodic oscillation frequencies of low-mass X-ray binaries

We analyzed the recently published kHz quasi-period oscillaiton (QPO) data in the neutron star low-mass X-ray binaries (LMXBs), in order to investigate the different correlations of the twin-peak kHz QPOs in bright Z sources and in the less luminous Atoll sources. We find a power-law relation nu(1)similar to nu(b)(2) between the upper and the lower kHz QPOs with different indices: b similar or equal to 1.5 for the Atoll source 4U 1728-34 and b similar or equal to 1.9 for the Z source Sco X-1. The implications of our results for the theoretical models for kHz QPOs are discussed.

Overview of QPOs in neutron-star low-mass X-ray binaries

Some aspects of the rapid X-ray variability of low magnetic-field neutron stars in low-mass X-ray binaries are briefly summarized.

A Model for Twin Kilohertz Quasi-periodic Oscillations in Neutron Star Low-Mass X-Ray Binaries

We suggest a plausible interpretation for the twin kiloHertz quasi-periodic oscillations (kHz QPOs) in neutron star low-mass X-ray binaries. We identify the upper kHz QPO frequencies to be the rotational frequency and the lower kHz QPOs the standing kink modes of loop oscillations at the inner edge of the accretion disk, respectively. Taking into account the interaction between the neutron star magnetic field and the disk, this model naturally relates the twin QPO frequencies with the star's spin frequencies. We have applied the model to four X-ray sources with kHz QPOs detected simultaneously and known spin frequencies.

The bottom magnetic field and magnetosphere evolution of neutron star in low-mass X-ray binary

The accretion-induced neutron star (NS) magnetic field evolution is studied through considering the accretion flow to drag the field lines aside and dilute the polar-field strength, and as a result the equatorial field strength increases, which is buried inside the crust on account of the accretion-induced global compression of star crust. The main conclusions of model are as follows: (i) the polar field decays with increase in the accreted mass; (ii) the bottom magnetic field strength of about 10 8 G can occur when the NS magnetosphere radius approaches the star radius, and it depends on the accretion rate as ˙ M 1/2 ; and (iii) the NS magnetosphere radius decreases with accretion until it reaches the star radius, and its evolution is little influenced by the initial field and the accretion rate after accreting ∼0.01 M� , which implies that the magnetosphere radii of NSs in low-mass X-ray binaries would be homogeneous if they accreted the comparable masses. As an extension, the physical effects of the possible strong magnetic zone in the X-ray NSs and recycled pulsars are discussed. Moreover, the strong magnetic fields

Quasi-Periodic Oscillations Resonantly Induced on Spin-Induced Deformed-Disks of Neutron Stars

We consider the situation where a spin-induced wavy perturbation is present on disks of neutron-star low-mass X-ray binaries. By non-linearly coupling with the perturbation, some modes of disk oscillations resonantly interact with the disks. We examine the resonant conditions under the assumption that the disks are vertically isothermal. The results show that when the spin frequency has some particular values, resonant coupling occurs at some radii. The modes and frequencies of these resonant oscillations are summarized. Some of these resonant oscillations seem to explain the frequencies of the observed kHz QPOs in objects of which the spin frequency is known.

A Lack of Radio Emission from Neutron Star Low‐Mass X‐Ray Binaries

We report strict upper limits to the radio luminosities of three neutron star low-mass X-ray binaries obtained with the Very Large Array while they were in hard X-ray states as observed with the Rossi X-Ray Timing Explorer: 1E 1724-307, 4U 1812-12, and SLX 1735-269. We compare these upper limits to the radio luminosities of several black hole binaries in very similar hard states and find that the neutron star systems are as faint as or fainter than all of the black hole candidates. The differences in luminosities can partly be attributed to the lower masses of the neutron star systems, which on theoretical and observational grounds are expected to decrease the radio luminosities as M0.8. However, there still remains a factor of 30 scatter in the radio luminosities of black hole and neutron star X-ray binaries, particularly at X-ray luminosities of a few percent Eddington. We find no obvious differences in the X-ray timing and spectral properties that can be correlated with the radio luminosity. We discuss the implications of these results on current models for the relationship between accretion and jets.

The distribution of kHz QPO frequencies in bright low mass X-ray binaries

We analyzed all published frequencies, and , of the twin kilohertz quasi-periodic oscillations (kHz QPOs) in bright neutron star low-mass X-ray binaries. The two frequencies are well correlated but, contrary to recent suggestions, the frequency–frequency correlation is significantly different from a relation. To check whether, although not following the the 3/2 relation, the QPO frequencies cluster around a region where , we re-analyzed the Sco X-1 data that were used to report that ratio and show that, because the distribution of ratios of linearly correlated measurements is intrinsically biased, although the significance of the clustering around previously reported in the case of Sco X-1 is formally correct, it does not provide any useful information about a possible underlying resonance mechanism in this source. Using the same data, we then show that the (unbiased) distribution of QPO frequencies is consistent with a uniform distribution at a level. To investigate this further, we analyzed a larger data set of Sco X-1 and four other sources, 4U 1608–52, 4U 1636–53, 4U 1728–34 and 4U 1820–30. We find that for all five sources the distribution of the kHz QPO frequencies is not uniform and has multiple peaks, which have no analogy in the distribution of points in the spectral color–color diagrams of these sources. Finally, we demonstrate that a simple random walk of the QPO frequencies can reproduce qualitatively the observed distributions in frequency and frequency ratio. This result weakens the support for resonance models of kHz QPOs in neutron stars.

Sidebands to the lower kilohertz quasi-periodic oscillation in 4U 1636-53

In this paper we report on further observations of the third and fourth kilohertz quasi-periodic oscillations (QPOs) in the power spectrum of the low-mass X-ray binary (LMXB) 4U 1636-53. These kilohertz QPOs are sidebands to the lower kilohertz QPO. The upper sideband has a frequency 55.5 +/- 1.7Hz larger than that of the contemporaneously measured lower kilohertz QPO. Such a sideband has now been measured at a significance >6sigma in the power spectra of three neutron-star LMXBs (4U 1636-53, 1728-34 and 1608-52). We also confirm the presence of a sideband at a frequency ~55Hz less than the frequency of the lower kilohertz QPO. The lower sideband is detected at a 3.5sigma level only when the lower kilohertz QPO frequency is between 800 and 850Hz. In that frequency interval, the sidebands are consistent with being symmetric around the lower kilohertz QPO frequency. The upper limit to the rms amplitude of the lower sideband is significantly lower than that of the upper sideband for lower kilohertz QPO frequencies >850Hz. Symmetric sidebands are unique to 4U 1636-53. This might be explained by the fact that lower kilohertz QPO frequencies as high as 800-850Hz are rare for 4U 1728-34 and 1608-52. Finally, we also measured a low-frequency QPO at a frequency of ~43Hz when the lower kilohertz QPO frequency is between 700 and 850Hz. A similar low-frequency QPO is present in the power spectra of the other two systems for which a sideband has been observed. We briefly discuss the possibility that the sideband is caused by Lense-Thirring precession.

M31 Globular Cluster X‐Ray Sources:XMM‐NewtonandChandraResults

We present the results of an M31 globular cluster (GC) X-ray source survey, based on the data of XMM-Newton and Chandra observations covering ~6100 arcmin2 of M31. We detected 43 X-ray sources coincident with GC candidates from various optical surveys. The inferred isotropic X-ray luminosities of GC sources lie between ~1035 and ~1039 ergs s-1 in the 0.3-10 keV energy band. The spectral properties of the 31 brightest sources in our sample were found to be similar to those of the low-mass X-ray binaries (LMXBs) located in the bulge and the GCs of the Milky Way. The spectral distribution of the M31 GC X-ray sources is consistent with that derived for the bulge of M31 and other nearby galaxies of different morphological type. Several sources demonstrate a correlation between the level of X-ray flux and the hardness of their energy spectrum reminiscent of the Galactic Z and atoll sources. We found that ~80% of the M31 GC sources with multiple flux measurements available show significant variability on timescales from days to years. The X-ray source RX J0043.2+4127, coincident with GC Bo 163, has been found to show recurrent transient outbursts with peak luminosities of ~1038 ergs s-1. Several sources in our sample show significant variability on a timescale of individual observations, ranging from aperiodic fluctuations to regular dipping. The X-ray luminosity function of GC sources is found to be significantly different from that of the point sources in the bulge and disk of M31. The luminosity distribution of M31 GC sources has ~10 times higher peak luminosity and a much higher fraction of bright sources than the Milky Way GC distribution. Six persistent sources in our sample (or ~14% of the total number) have luminosities exceeding 1038 ergs s-1 during all observations, and three other sources occasionally exceed that luminosity level. Our observations indicate that GC sources make the dominant contribution to the bright source counts in the areas of M31 covered by the survey: ~40% of the total number of sources with luminosities above 1037 ergs s-1 reside in GCs, with the fraction of GC sources rising to 67%-90% for luminosities above 1038 ergs s-1. The contribution of the GC sources to the total number of bright sources found in M31 is much higher than in the Milky Way but surprisingly close to that of early-type galaxies. We found that the brightest M31 GC sources tend to reside at large galactocentric distances outside the central bulge. We found that GCs hosting bright X-ray sources are optically brighter and more metal-rich than the rest of the M31 GCs, in agreement with previous studies. The brightest sources, with luminosities above ~1038 ergs s-1, show a tendency to reside in more metal-poor clusters. The remarkable similarities between the properties of the M31 GC X-ray sources and those of Galactic neutron star LMXBs lead us to expect most of the persistent M31 GC X-ray sources to be LMXB systems with neutron star primaries. However, the current X-ray spectral and timing data cannot rule out the possibility of finding active accreting black holes in our GC source sample.

The distances to Galactic low-mass X-ray binaries: consequences for black hole luminosities and kicks

We investigated the reported distances of Galactic black hole (BH) and neutron star low‐mass X‐ray binaries (LMXBs). Comparing the distances derived for the neutron stars Cyg X–2 and XTE J2123–058 using the observed Eddington limited photospheric radius expansion bursts with the distances derived using the observed radius and effective temperature of the companion star, we find that the latter are smaller by approximately a factor of 1.5–2. The latter method is often employed to determine the distance to BH LMXBs. A possible explanation for this discrepancy is that the stellar absorption lines in fast rotating companion stars are different from those in the slowly rotating template stars as was found before for early‐type stars. This could lead to a systematic mis‐classification of the spectral type of the companion star, which in turn would yield a systematic error in the distance. Further, we derive a distance of 4.0+2.0−1.2 kpc for V404 Cyg, using parameters available in the literature. The interstellar extinction seems to have been overestimated for XTE J1550–564 and possibly for two other BH sources (H 1705–25 and GS 2000+25) as well. As a result of this, the distance to XTE J1550–564 may have been underestimated by as much as a factor three. We find that, using the new distances for XTE J1550–564 and V404 Cyg, the maximum outburst luminosity for at least five, but perhaps even seven, of the 15 BH soft X‐ray transients exceed the Eddington luminosity for a 10‐M⊙ BH – showing that these systems would be classified as ultra‐luminous X‐ray sources had we observed them in other Galaxies. This renders support for the idea that many ultra‐luminous X‐ray sources are stellar‐mass rather than intermediate‐mass BHs. We find that the rms‐value of the distance to the Galactic plane for BHs is consistent with that of neutron star LMXBs. This suggests that BHs could also receive a kick‐velocity during their formation, although this has to be investigated in more detail. We find that the Galactic neutron star and BH l‐ and b‐distributions are consistent with being the same. The neutron star and BH distribution is asymmetric in l with an excess of systems between −30° < l < 0° over systems with 0° < l < 30°.

New results on transient neutron-star low-mass X-ray binaries in quiescence

We review the main observational properties of neutron star low mass transients in quiescence. We first survey the discoveries of BeppoSAX. We then focus on recent discoveries by Chandra and XMM-Newton, with special emphsasis on the detection of the quiescent counterpart of SAX J1808.4-3658 and the study of variability in the quiescent state.

The Discovery of Optical Bursts from MS 1603.6+2600 (UW Coronae Borealis)

We report the discovery of several optical burstlike events from the low-mass X-ray binary MS 1603.6+2600 (UW CrB). The events last for a few tens of seconds, exhibit a very fast rise and slow decay, and involve optical brightening of a factor of 2-3. The flares appear distinct from the lower level flickering and instead strongly resemble reprocessed type I X-ray bursts as seen in a number of other neutron star low-mass X-ray binaries. In conjunction with the previously reported candidate X-ray burst, these confirm that the compact object in UW CrB is a neutron star. We examine the optical burst brightness and recurrence times and discuss how the nature of the system can be constrained. We conclude that the source is most likely an accretion disk corona source at an intermediate distance, rather than a nearby quiescent system or very distant dipper.

Crust-core coupling and r-mode damping in neutron stars: a toy model

r-modes in neutron stars with crusts are damped by viscous friction at the crust–core boundary. The magnitude of this damping, evaluated by Bildsten & Ushomirsky (BU) under the assumption of a perfectly rigid crust, sets the maximum spin frequency for neutron stars spun up by accretion in low-mass X-ray binaries (LMXBs). In this paper we explore the mechanical coupling between the core r-modes and the elastic crust, using a toy model of a constant-density neutron star having a crust with a constant shear modulus. We find that, at spin frequencies in excess of ≈50 Hz, the r-modes strongly penetrate the crust. This reduces the relative motion (slippage) between the crust and the core compared with the rigid-crust limit. We therefore revise down, by as much as a factor of 102–103, the damping rate computed by BU, significantly reducing the maximal possible spin frequency of neutron stars with solid crusts. The dependence of the crust–core slippage on the spin frequency is complicated, and is very sensitive to the physical thickness of the crust. If the crust is sufficiently thick, the curve of the critical spin frequency for the onset of the r-mode instability becomes multivalued for some temperatures; this is related to avoided crossings between the r-mode and higher-order torsional modes in the crust. The critical frequencies are comparable to the observed spins of neutron stars in LMXBs and millisecond pulsars.