Star Low-mass X-ray Binaries Research Articles

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.

XMM-Newton observations of faint X-ray sources in globular clusters

Using the new generation of X-ray observatories, we are now beginning to identify populations of close binaries in globular clusters, previously elusive in the optical domain because of the high stellar density. These binaries are thought to be, at least in part, responsible for delaying the inevitable core collapse of globular clusters and their identification is therefore essential in understanding the evolution of globular clusters, as well as being valuable in the study of the binaries themselves. Here, we present observations made with XMM-Newton of six globular clusters, in which we have identified neutron star low mass X-ray binaries and their descendants (millisecond pulsars), cataclysmic variables and other types of binaries. We discuss not only the characteristics of these binaries, but also their formation and evolution in globular clusters and their use in tracing the dynamical history of these clusters.

Production of QPOs in accreting neutron star systems

Kilohertz QPOs have been detected from more than 20 neutron stars in low-mass X-ray binaries. Several different ideas have been proposed for their generation, involving resonances, magnetic interactions, and sharp transitions in the accretion flow. We show that although details are uncertain at this time, it is clear that the stellar magnetic field has a dynamic influence on the accretion flow. We also discuss the inferences about dense matter and strong gravity that can be drawn from all models, and the qualitative advances expected with a future X-ray timing mission.

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.

Infrared Spectroscopy of Symbiotic Stars. IV. V2116 Ophiuchi/GX 1+4, The Neutron Star Symbiotic

We have computed, using 17 infrared radial velocities, the first set of orbital elements for the M giant in the symbiotic binary V2116 Ophiuchi. The giant's companion is a neutron star, the bright X-ray source GX 1+4. We rule out the previously proposed period of 304 days and instead find an orbital period of 1161 days, by far the longest of any known X-ray binary. The orbit has a modest eccentricity of 0.10, with an orbital circularization time of ≲5 × 106 yr. The large mass function of the orbit significantly restricts the mass of the M giant. Adopting a neutron star mass of 1.35 M☉, the maximum mass of the M giant is 1.22 M☉, making it the less massive star. Spectrum synthesis analysis of several infrared spectral regions results in slightly subsolar abundances for most metals. Carbon and nitrogen are in the expected ratio resulting from the red-giant first dredge-up phase. The lack of 17O suggests that the M giant has a mass less than 1.3 M☉, consistent with our maximum mass. The surface gravity and maximum mass of the M giant result in a radius of 103 R☉, much smaller than its estimated Roche lobe radius. Thus, the mass loss of the red giant is via a stellar wind. These properties argue that the M giant is near the tip of the first-ascent giant branch. Although the M-giant companion to the neutron star has a mass similar to the late-type star in low-mass X-ray binaries, its near-solar abundances and apparent runaway velocity are not fully consistent with the properties of this class of stars. Thus, in many ways this symbiotic X-ray binary system is unique, and various scenarios for its possible evolution are discussed.

Superbursts from Strange Stars

Recent models of carbon ignition on accreting neutron stars predict superburst ignition depths that are an order of magnitude larger than those observed. We explore a possible solution to this problem, that the compact stars in low-mass X-ray binaries that have shown superbursts are in fact strange stars with a crust of normal matter. We calculate the properties of superbursts on strange stars and the resulting constraints on the properties of strange quark matter. We show that the observed ignition conditions exclude fast neutrino emission in the quark core, for example, by the direct Urca process, which implies that strange quark matter at stellar densities should be in a color superconducting state. For slow neutrino emission in the quark matter core, we find that reproducing superburst properties requires a definite relation between three poorly constrained properties of strange quark matter: its thermal conductivity (K), its slow neutrino emissivity (ν QνT), and the energy released by converting a nucleon into strange quark matter (QSQM).

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.

Modelling the spin equilibrium of neutron stars in low-mass X-ray binaries without gravitational radiation

In this paper we discuss the spin equilibrium of accreting neutron stars in low-mass X-ray binaries (LMXBs). We demonstrate that, when combined with a naive spin-up torque, the observed data lead to inferred magnetic fields which are at variance with those of Galactic millisecond radio pulsars. This indicates the need for either additional spin-down torques (e.g. gravitational radiation) or an improved accretion model. We show that a simple consistent accretion model can be arrived at by accounting for radiation pressure in rapidly accreting systems (above a few per cent of the Eddington accretion rate). In our model the inner disc region is thick and significantly sub-Keplerian and the estimated equilibrium periods are such that the LMXB neutron stars have properties that accord well with the Galactic millisecond radio pulsar sample. The implications for future gravitational-wave observations are also discussed briefly.

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.

Latitudinal Shear Instabilities during Type I X‐Ray Bursts

Coherent oscillations have been observed during thermonuclear (type I) X-ray bursts from 14 accreting neutron stars in low-mass X-ray binaries, providing important information about their spin frequencies. However, the origin of the brightness asymmetry on the neutron star surface producing these oscillations is still not understood. We study the stability of a zonal shearing flow on the neutron star surface using a shallow-water model. We show that differential rotation of 2% between the pole and equator, with the equator spinning faster than the poles, is unstable to hydrodynamic shear instabilities. The unstable eigenmodes have a low azimuthal wavenumber m, wave speeds 1% or 2% below the equatorial spin rate, and e-folding times 1 s. Instability is related to low-frequency, buoyantly driven r-modes that have a mode frequency within the range of rotation frequencies in the differentially rotating shell. In addition, a modified Rayleigh's criterion based on potential vorticity rather than vorticity must be satisfied. The eigenfunctions of the unstable modes have the largest amplitudes nearer to the rotational pole than the equator. We discuss the implications for burst oscillations. Growth of shear instabilities may explain the brightness asymmetry in the tails of X-ray bursts. However, some fine-tuning of the level of differential rotation and a spin frequency near 300 Hz are required in order for the fastest growing mode to have m = 1. If shear instabilities are to operate during a burst, temperature contrasts of 30% across the star must be created during ignition and spreading of the flash. The frequency drift may be related to the nonlinear development of the instability as well as the cooling of the burning layer. The properties and observability of the unstable modes vary strongly with neutron star rotation frequency.

XMM-Newton detection of type I X-ray bursts in M 31

We searched for X-ray bursts in XMM-Newton archival data of X-ray sources in M 31 globular clusters (GCs) and GC candidates. We detected two bursts simultaneously in EPIC pn and MOS detectors and some more candidates in EPIC pn. The energy distribution of the burst photons and the intrinsic luminosity during the peak of the bursts indicate that at least the strongest burst was a type I radius expansion burst. The bursts identify the sources as neutron star low mass X-ray binaries in M 31. The type I X-ray bursts in M 31 are the first detected outside the Milky Way and show that with the help of XMM-Newton X-ray bursts can be used to classify neutron star low mass X-ray binaries in Local Group galaxies.

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.

Using X-Rays to Probe the Compact Binary Content of Globular Clusters

AbstractGlobular clusters (GCs) harbour a large number of close binaries which are hard to identify optically due to high stellar densities. Observing these GCs in X-rays, in which the compact binaries are bright, diminishes the over-crowding problem. Using the new generation of X-ray observatories, it is possible to identify populations of neutron star low mass X-ray binaries, cataclysmic variables and millisecond pulsars as well as other types of binaries. We present the spectra of a variety of binaries that we have identified in four GCs observed by XMM-Newton. We show that through population studies we can begin to understand the formation of individual classes of binaries in GCs and hence start to unfold the complex evolutionary paths of these systems.

Millisecond pulsars with r-modes as steady gravitational radiators.

Millisecond pulsars (MSPs) probably achieve their fast rotation by mass transfer from their companion stars in low-mass x-ray binaries (LMXBs). The lack of MSPs and LMXBs rotating near breakup has been attributed to the accretion torque being balanced, at fast rotation, by gravitational radiation, perhaps caused by an unstable oscillation mode. It has been argued that internal dissipation involving hyperons may cause LMXBs to evolve into a quasisteady state, with nearly constant rotation rate, temperature, and mode amplitude. We show that MSPs descending from these LMXBs spend a long time in a similar state, as extremely steady sources of gravitational waves and thermal x rays, while they spin down due to gravitational radiation and the standard magnetic torque. Observed MSP braking torques already place meaningful constraints on this scenario.

FirstINTEGRALobservations of eight persistent neutron star low mass X-ray binaries

Early results from the INTEGRAL Core Program, for a sample of eight persistently bright neutron star low mass X-ray binaries in the energy range from 5 keV to 200 keV, are presented. It is shown that INTEGRAL efficiently detects sources and that spectra may be obtained up to several hundreds of keV by combining data from three of the four INTEGRAL instruments: JEM-X, IBIS and SPI. For the source GX 17+2 it is shown that the spectrum extends well above 100 keV with a flattening above 30 keV. This might suggest a non-thermal comptonisation emission, but uncertainties in the current data reduction and background determination do not allow firm conclusions to be drawn yet.

Quark matter in compact stars

This paper discusses possible astrophysical signals by means of which quark matter may register itself in isolated millisecond pulsars and neutron stars in low-mass x-ray binaries. Implications of absolutely stable strange quark matter fow white dwarfs are pointed out in the last section.

The Amplitude Evolution and Harmonic Content of Millisecond Oscillations in Thermonuclear X‐Ray Bursts

We present a comprehensive observational and theoretical analysis of the amplitudes and profiles of oscillations that occur during thermonuclear X-ray bursts from weakly magnetized neutron stars in low-mass X-ray binaries. Our sample contains 59 oscillations from six sources observed with the Rossi X-Ray Timing Explorer. The oscillations that we examined occurred primarily during the decaying portions of bursts and lasted for several seconds each. We find that the oscillations are as large as 15% during the declines of the bursts and that they appear and disappear because of intrinsic variations in their fractional amplitudes. However, the maxima in the amplitudes are not related to the underlying flux in the burst. We derive folded profiles for each oscillation train to study the pulse morphologies. The mean rms amplitudes of the oscillations are 5%, although the eclipsing source MXB 1659-298 routinely produces 10% oscillations in weak bursts. We also produce combined profiles from all of the oscillations from each source. Using these pulse profiles, we place upper limits on the fractional amplitudes of harmonic and half-frequency signals of 0.3% and 0.6%, respectively (95% confidence). These correspond to less than 5% of the strongest signal at integer harmonics and less than 10% of the main signal at half-integer harmonics. We then compare the pulse morphologies with theoretical profiles from models with one or two antipodal bright regions on the surface of a rotating neutron star. We find that if one bright region is present on the star, it must either lie near the rotational pole or cover nearly half the neutron star in order to be consistent with the observed lack of harmonic signals. If an antipodal pattern is present, the hot regions must form very near the rotational equator. We discuss how these geometric constraints challenge current models for the production of surface brightness variations during the cooling phases of X-ray bursts.