Oscillations In Low-mass X-ray Binaries Research Articles

A possible origin of kilohertz quasi-periodic oscillations in low-mass X-ray binaries

Abstract A possible origin of kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries is proposed. Recent numerical magnetohydrodynamic simulations of accretion disks with turbulent magnetic fields of magneto-rotational instability definitely show the presence of two-armed spiral structure in the quasi-steady state of accretion disks. In such deformed disks, two-armed ($m=2$) c-mode ($n=1$) oscillations are excited by wave–wave resonant instability. Among these excited oscillations, the fundamental in the radial direction ($n_{\rm r}=0$) will be the higher kHz QPO of twin QPOs, and the first overtone ($n_{\rm r}=1$) in the radial direction will be the lower kHz QPO of the twin. A possible cause of the twin high-frequency QPOs in black hole X-ray binaries is also discussed in the final section.

Extracting multipole moments of neutron stars from quasi-periodic oscillations in low mass X-ray binaries

We consider the kilohertz quasi-periodic oscillations of low-mass X-ray binaries within the Hartle-Thorne spacetime. We show that the interpretation of the epicyclic frequencies of this spacetime with the observed kilohertz quasi-periodic oscillations, within the Relativistic Precession Model, allows us to extract the total mass M, angular momentum J, and quadrupole moment Q of the compact object in a low-mass X-ray binary. We exemplify this fact by analyzing the data of the Z-source GX 5-1. We show that the extracted multipole structure of the compact component of this source deviates from the one expected from a Kerr black hole and instead it points to a neutron star explanation.

What can we extract from quasiperiodic oscillations?

In light of the relativistic precession model, we present here detailed analyses, extending the ones performed in the Schwarzschild and Kerr spacetimes. We consider the kilohertz quasi-periodic oscillations in the Hartle-Thorne spacetime, which describes the gravitational field of a rotating and deformed object. We derive the analytic formulas for the epicyclic frequencies in the Hartel-Thorne spacetime and by means of these frequencies we interpret the kilohertz quasi-periodic oscillations of low-mass X-ray binaries of the atoll and Z - sources, on the basis of the relativistic precession model. Particularly we perform analyzes for Z -source: GX 5-1. We show that the quasi-periodic oscillations data can provide information on the parameters, namely, the mass, angular momentum and quadrupole moment of the compact objects in the low-mass X-ray binaries.

Orbital and epicyclic frequencies around rapidly rotating compact stars in scalar-tensor theories of gravity

We study the orbital and epicyclic frequencies of particles orbiting around rapidly rotating neutron stars and strange stars in a particular scalar-tensor theory of gravity. We find very large deviations of these frequencies, when compared to their corresponding values in general relativity, for the maximum-mass rotating models. In contrast, for models rotating with spin frequency of 700Hz (approximately the largest known rotation rate of neutron stars), the deviations are generally small. Nevertheless, for a very stiff equation of state and a high mass the deviation of one of the epicyclic frequencies from its GR value is appreciable even at a spin frequency of 700Hz. In principle, such a deviation could become important in models of quasi-periodic oscillations in low-mass x-ray binaries and could serve as a test of strong gravity (if other parameters are well constraint). Even though the present paper is concentrated mainly on orbital and epicyclic frequencies, we present here for the first time rapidly rotating, scalarized equilibrium compact stars with realistic hadronic equations of state and strange matter equation of state. We also provide analytical expressions for the exterior spacetime of scalarized neutron stars and their epicyclic frequencies in the nonrotating limit.

The magnetohydrodynamics model of twin kilohertz quasi-periodic oscillations in low-mass X-ray binaries

We suggest an explanation for the twin kilohertz quasi-periodic oscillations (kHz QPOs) in low-mass X-ray binaries (LMXBs) based on magnetohydrodynamics (MHD) oscillation modes in neutron star magnetospheres. Including the effect of the neutron star spin, we derive several MHD wave modes by solving the dispersion equations, and propose that the coupling of the two resonant MHD modes may lead to the twin kHz QPOs. This model naturally relates the upper, lower kHz QPO frequencies with the spin frequencies of the neutron stars, and can well account for the measured data of six LMXBs.

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.

On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries

I study the behaviour of the maximum rms fractional amplitude, $r_{\rm max}$ and the maximum coherence, $Q_{\rm max}$, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that: (i) The maximum rms amplitudes of the lower and the upper kHz QPO, $r^{\ell}_{\rm max}$ and $r^{\rm u}_{\rm max}$, respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower kHz QPO, $Q^{\ell}_{\rm max}$, first increases and then decreases exponentially with luminosity, at a faster rate than both $r^{\ell}_{\rm max}$ and $r^{\rm u}_{\rm max}$; (iii) the maximum coherence of the upper kHz QPO, $Q^{\rm u}_{\rm max}$, is more or less independent of luminosity; and (iv) $r_{\rm max}$ and $Q_{\rm max}$ show the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both $r_{\rm max}$ and $Q_{\rm max}$ in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star. I discuss possible interpretations of these results in terms of the modulation mechanisms that may be responsible for the observed variability.

A Resonance Model of Quasi-Periodic Oscillations of Low-Mass X-Ray Binaries

Abstract We try to understand the quasi-periodic oscillations (QPOs) in low-mass neutron-star and black-hole X-ray binaries by a resonance model in warped disks with precession. Our main concern is high-frequency QPOs, hectohertz QPOs, and horizontal-branch QPOs in the z sources and the atoll sources, and the corresponding QPOs in black-hole X-ray binaries. Our resonance model can qualitatively, but systematically, explain these QPOs by regarding hectohertz QPOs as a precession of warp.

Phase Transitions in Neutron Stars and Maximum Masses.

Using the most recent realistic effective interactions for nuclear matter with a smooth extrapolation to high densities including causality, we constrain the equation of state and calculate maximum masses of rotating neutron stars. First- and second-order phase transitions to, e.g., quark matter at high densities are included. If neutron star masses of approximately 2.3 M middle dot in circle from quasi-periodic oscillations in low-mass X-ray binaries are confirmed, a soft equation of state as well as strong phase transitions can be excluded in neutron star cores.

Quasi-periodic oscillations in low-mass X-ray binaries and constraints on the equation of state of neutron star matter

Recently discovered quasi-periodic oscillations in the X-ray brightness of low-mass X-ray binaries are used to derive constraints on the mass of the neutron star component and the equation of state of neutron star matter. The observations are compared with models of rapidly rotating neutron stars which are calculated by means of an exact numerical method in full relativity. For the equations of state we select a broad collection of models representing different assumptions about the many-body structure and the complexity of the composition of superdense matter. The mass constraints differ from their values in the approximate treatment by ∼10 per cent. Under the assumption that the maximum frequency of the quasi-periodic oscillations originates from the innermost stable orbit, the mass of the neutron star is in the range M∼1.92ߝ2.25 M⊙. The quasi-periodic oscillation in the Atoll-source 4U 1820-30 in particular is only consistent with equations of state that are rather stiff at high densities, which is explainable, so far, only with pure nucleonic/leptonic composition. This interpretation contradicts the hypothesis that the protoneutron star formed in SN 1987A collapsed to a black hole, since this would demand a maximum neutron star mass below 1.6 M⊙. The recently suggested identification of quasi-periodic oscillations with frequencies of about 10 Hz with the Lense—Thirring precession of the accretion disc is found to be inconsistent with the models studied in this work, unless it is assumed that the first overtone of the precession is observed.

KHz Quasiperiodic Oscillations in Low-Mass X-Ray Binaries as Probes of General Relativity in the Strong-Field Regime

We consider the interpretation of a pair of kHz quasiperiodic oscillations (QPOs) in the Fourier spectra of two low-mass x-ray binaries, Sco X-1 and 4U1608-52, hosting an old accreting neutron star. The observed frequency difference of these QPOs decreases as their frequency increases, contrary to simple beat frequency models. We show that the behavior of these QPOs is instead well matched in terms of the fundamental frequencies for test particle motion in the gravitational field of the neutron star, for reasonable star masses, and nearly independent of the star spin. These results are not reproduced through the post--Newtonian approximation of general relativity. kHz QPOs from x-ray binaries likely provide an accurate laboratory for strong-field general relativity.

R. Buccheri, J. van Paradijs, M. A. Alpar, The Many of Neutron Stars

We see neutron stars principally by their radio and X-ray emission. Their appearance in these different bands depends on whether the emission comes from the surface or its magnetosphere. New phenomena continue to be found from neutron stars, which makes it an exciting and topical research area. This volume is a collection of the papers from a NATO Advanced Study Institute held in Italy in October 1996. Many, and for me the most interesting ones, are substantial reviews on topics such as Pulsar magnetic fields and glitches (M. Ruderman), Radio pulsar population properties (D. Lorimer), Gamma-ray emission from CGRO pulsars (G. Kanbach), Neutron stars and black holes in X-ray binaries (J. van Paradijs), Kilohertz quasi-periodic oscillations in low-mass X-ray binaries (M. van der Klis), Thermonuclear burning on rapidly accreting neutron stars (L. Bildsten), On the X-ray emission properties of rotation powered pulsars (W. Becker and J. Truemper). It will serve as a useful reference and source book for students in high energy astrophysics and related fields. The high price may deter its purchase by individuals, but it will be a good volume for a library needing recent coverage on neutron stars. It does not of course include the most recent developments on anomalous X-ray pulsars or magnetars.

Rossi X-ray timing explorer observations of kilohertz QPO

I review the RXTE observations f the recently discovered kilohertz quasi-periodic oscillations in low-mass X-ray binaries. A table is presented with all kHz QPO discoveries up to December 10, 1997.

Lense-Thirring Precession and Quasi-periodic Oscillations in Low-Mass X-Ray Binaries

Relativistic dragging of inertial frames around fast-rotating collapsed stars is substantial and can give rise to observable effects. We consider kHz quasi-periodic oscillation (QPO) sources, low-mass X-ray binaries (LMXRBs) containing an accreting neutron star. Within beat frequency models, both the Keplerian frequency of the innermost region of the accretion disk (~0.3-1.2 kHz) and the neutron star spin frequency (~0.3-0.4 kHz) are directly observed. From these, the Lense-Thirring precession frequency (tens of Hz) of the same material as gives rise to the kHz QPOs is determined within a factor of ~4, depending on the neutron star equation of state. The classical contribution from neutron star oblateness decreases the precession frequency slightly. The broad peaks at frequencies ~20-40 Hz in the power spectra of the Atoll sources 4U 1728-34, 4U 0614+091, and KS 1731-260 and their variations with the higher kHz QPO frequency are well matched by Lense-Thirring precession of material in the innermost disk region. We also suggest that the ~15-50 Hz horizontal branch QPOs of GX 5-1 and GX 17+2 (and likewise other Z-type LMXRBs) arise from the same mechanism.

Kilohertz quasi-periodic oscillations in low-mass X-ray binaries

I review the current observational status of the new kilohertz quasi-periodic oscillations in accreting lowmagnetic field neutron stars, and critically discuss some of the proposed models.

Beat frequency modulation of T Tauri accretion rates

A general model of magnetically controlled accretion onto T Tauri stars is presented. In this model the magnetic field is oriented arbitrarily in relation to the star's rotation axis. The resultant interplay between the magnetic field and accretion disc causes a variable accretion rate. The dominant timescale of this variability is the beat frequency between the stellar rotation frequency and the orbital frequency at the magnetosphere boundary. This model is analogous to that developed to explain quasi-periodic oscillations in low-mass X-ray binaries.

Low-frequency quasi-periodic oscillations in low-mass X-ray binaries and galactic black hole candidates

We consider the inner regions of accretion disks surrounding black holes and neutron stars and investigate the nonlinear time-dependent evolution of thermal-viscous instabilities. The viscous stress is assumed to be proportional to the gas pressure with the viscosity parameter formulated as alpha = min alpha (sub zero) (h/r) (exp n), alpha(sub max), where h is the local scale height, r is the distance from the central compact object, and n, alpha(sub zero) and alpha(sub max) are constants. It is found that the disk is unstable for alpha sufficiently sensitive to h (n greater than or equal to 1.2). The instabilities are globally coherent in the entire unstable region of the disk, and, depeding on the viscosity parameters, the time variability of the mass accretion rates are manifested as periodic or quasi-periodic oscillations. We show that, the low-frequency (approximately 0.04 Hz) quasi-periodic oscillations (QPOs) discovered recently in some of the black hole candidates (Cyg X-1 and GRO J0422+32) and a low-mass X-ray binary (Rapid Burster MXB 1730-335) may be explicable by the thermal-viscous instabilities in accretion disks. The observations of QPOs place constraints on the viscosity parameters and suggest that (n, alpha(sub zero) approximately (1.6, 30) for the Rapid Burster with a 1.4 solar mass neutron star. In the case of black hole candidates, the dependence of alpha on h/r is less steep corresponding to n approximately 1.2-1.3 for black holes less than 10 solar mass.

Energy dependence of normal branch quasi-periodic intensity oscillations in low-mass X-ray binaries

The properties of the approximately 6 Hz quasi-periodic X-ray intensity oscillations observed in the low-mass X-ray binary Cyg X-2 when it is on the normal spectral branch are shown to be consistent with a model in which photons from a central source with a fixed spectrum are Comptonized by an oscillating radial inflow. As the electron scattering optical depth of the flow varies, the spectrum of the escaping X-rays appears to rotate about a pivot energy that depends mainly on the electron temperature in the flow. The temperature derived from the observed energy dependence of the Cyg X-2 normal branch oscillations is approximately 1 keV, in good agreement with the estimated Compton temperature of its X-ray spectrum. The mean optical depth tau of the Comptonizing flow is inferred to be about 10, while the change in tau over an oscillation is estimated to be about 1; both values are in good agreement with radiation hydrodcode simulations of the radial flow.

A review of quasi-periodic oscillations in low-mass X-ray binaries

A review of the quasi-periodic oscillations (QPO) observed in the X-ray flux of low-mass X-ray binaries is presented. In Section 1 a general background is given on galactic populations of accretion-driven X-ray sources. Section 2 contains a description of the methods that have been used to analyze these QPO. In Section 3 the QPO observations are described in some detail. Models for the QPO are considered in Section 4. In Section 5 the time lags observed between QPO at different photon energies and their possible implications for X-ray spectral models are discussed. Conclusions are summarized in Section 6.

Energy dependent delay measurements of quasi-periodic oscillations in low-mass X-ray binaries

Results of Monte Carlo simulations of the Comptonization of photons in a finite spherical Comptonizing cloud are presented. The energy dependence of the arrival times of the Comptonized photons is very different for different input photon spectra. If the delays observed in the high-frequency QPO of Cyg X-2 and GX5-1 are the result of Comptonization, multichannel time-delay measurements may constrain the energy of the input photons and thus provide important information on the accretion process in low-mass X-ray binaries.