Kilohertz QPOs Research Articles

Detection of thermonuclear X-ray bursts and dips from the X-ray binary 4U 1323-62 with AstroSat/LAXPC

Using data from the Large Area X-ray Proportional Counter (LAXPC) on the AstroSat satellite, we observed Type-1 thermonuclear X-ray bursts from the low mass X-ray binary neutron star 4U 1323–62. Observations of 4U 1323-62 that were carried out during the performance verification phase of the AstroSat satellite indicated six thermonuclear X-ray bursts in a total effective exposure of ∼49.5 ks for about two consecutive days. Recurrence time of the detected thermonuclear bursts is in accordance with the orbital period of the source, ∼9400 s. Moreover, the light curve of 4U 1323–62 revealed the presence of two dips. We present the results from time-resolved spectroscopy performed during all of the six X-ray bursts and also report the detection of a known low frequency quasi-periodic oscillation (LFQPO) at ∼1 Hz from the source. However, any evidence of kilohertz QPO was not found. We have shown the burst profile at different energy ranges. Assuming a distance of 10 kpc, we observed a mean flux of ∼1.8×10−9 erg cm2 s−1. The radius of the blackbody is found to be highly consistent with the blackbody temperature and the blackbody flux of the bursts.

Kilohertz QPOs in low-mass X-ray binaries as oscillation modes of tori around neutron stars – I

Kilohertz QPOs in low-mass X-ray binaries as oscillation modes of tori around neutron stars – I

KHz QPOs in LMXBs, relations between different frequencies and compactness of stars

It is suggested that the mass of four compact stars SAX J1808.4–3658, KS 1731–260, SAX J1750.8–2900 and IGR J17191–2821 can be determined from the difference in the observed kilohertz (kHz) QPO-s of these stars. The star radius is very close to the marginally stable orbit R ms predicted by Einstein’s general relativity. This serves as an evidence for the existence of strange stars with these four stars as examples. It may be noted that the first of these stars, at that time the unique millisecond X-ray pulsar with an accurate determination of its rotation period, was suggested to be a strange star from other considerations more than a decade back ( Li et al., 1999a). It showed kHz QPO-s 8 years back and so far only one set has been observed. This is the first time we give an estimate of the mass of the star using this set of the kHz QPO-s observed.

QPO constraints on neutron stars

The kilohertz frequencies of QPOs from accreting neutron star systems imply that they are generated in regions of strong gravity, close to the star. This suggests that observations of the QPOs can be used to constrain the properties of neutron stars themselves, and in particular to inform us about the properties of cold matter beyond nuclear densities. Here we discuss some relatively model-insensitive constraints that emerge from the kilohertz QPOs, as well as recent developments that may hint at phenomena related to unstable circular orbits outside neutron stars.

On the nature of the Cygnus X-2 like Z-track sources

Based on the results of applying the extended ADC emission model for low mass X-ray binaries to three Z-track sources: GX340+0, GX5-1 and CygX-2, we propose an explanation of the CygnusX-2 like Z-track sources. The Normal Branch is dominated by the increasing radiation pressure of the neutron star caused by a mass accretion rate that increases between the soft apex and the hard apex. The radiation pressure continues to increase on the Horizontal Branch becoming several times super-Eddington. We suggest that this disrupts the inner accretion disk and that part of the accretion flow is diverted vertically forming jets which are detected by their radio emission on this part of the Z-track. We thus propose that high radiation pressure is the necessary condition for the launching of jets. On the Flaring Branch there is a large increase in the neutron star blackbody luminosity at constant mass accretion rate indicating an additional energy source on the neutron star. We find that there is good agreement between the mass accretion rate per unit emitting area of the neutron star mdot at the onset of flaring and the theoretical critical value at which burning becomes unstable. We thus propose that flaring in the CygnusX-2 like sources consists of unstable nuclear burning. Correlation of measurements of kilohertz QPO frequencies in all three sources with spectral fitting results leads to the proposal that the upper kHz QPO is an oscillation always taking place at the inner accretion disk edge, the radius of which increases due to disruption of the disk by the high radiation pressure of the neutron star.

Simultaneous observations of a pair of kilohertz QPOs and a plausible 1860 Hz QPO from an accreting neutron star system

We report an indication (3.22 σ) of ≈ 1860 Hz quasi-periodic oscillations from a neutron star low-mass X-ray binary 4U 1636–536. If confirmed, this will be by far the highest frequency feature observed from an accreting neutron star system, and hence could be very useful in understanding such systems. This plausible timing feature was observed simultaneously with lower (≈ 585 Hz) and upper (≈ 904 Hz) kilohertz quasi-periodic oscillations. The two kilohertz quasi-periodic oscillation frequencies had a ratio of ∼ 1.5, and the frequency of the alleged ∼ 1860 Hz feature was close to the triple and the double values of these frequencies. This can be useful for constraining the models of all the three features. In particular, the ≈ 1860 Hz feature could be (1) from a new and heretofore unknown class of quasi-periodic oscillations, or (2) the first observed overtone of lower or upper kilohertz quasi-periodic oscillations. Finally, we note that, although the relatively low significance of the ≈ 1860 Hz feature argues for caution, even a 3.22 σ feature at such a uniquely high frequency should be interesting enough to spur a systematic search in the archival data, as well as to scientifically motivate sufficiently large timing instruments for the next generation X-ray missions.

Discovery of Kilohertz Quasi‐Periodic Oscillations and State Transitions in the Low‐Mass X‐Ray Binary 1E 1724−3045 (Terzan 2)

We have studied the rapid X-ray time variability in 99 pointed observations with the Rossi X-Ray Timing Explorer (RXTE) Proportional Counter Array of the low-mass X-ray binary 1E 1724-3045, including, for the first time, observations of this source in its island and banana states, confirming the atoll nature of this source. We report the discovery of kilohertz quasi-periodic oscillations (kHz QPOs). Although we have five detections of the lower kHz QPO and one detection of the upper kHz QPO, in none of the observations we detect both QPOs simultaneously. By comparing the dependence of the rms amplitude with energy of kHz QPOs in different atoll sources, we conclude that this information cannot be use to unambiguously identify the kilohertz QPOs as was previously thought. We find that Terzan 2 in its different states shows timing behavior similar to that seen in other neutron-star low-mass X-ray binaries (LMXBs). We studied the flux transitions observed between 2004 February and 2005 October and conclude that they are due to changes in the accretion rate.

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.

A Study on the Correlations between the Twin kHz QPO Frequencies in Sco X-1

For the bright neutron star low-mass X-ray binary Sco X-1, we analyzed all updated frequencies of the twin kilohertz quasi-periodic oscillations (kHz QPOs), their correlations and distributions. We found that the frequency separation of the kHz QPO peaks appears not to be a constant, rather, it decreases with increasing inferred mass accretion rate. We show that the currently available data of Sco X-1 by Rossi X-ray Timing Explorer are inconsistent with the proposals of the beat model that the frequency separation is a constant. Our conclusions are consistent with those of some previous researchers and we discuss further implications for the kilohertz QPO models.

Comparing Black Hole and Neutron Star Variability

There are remarkable similarities between the rapid X-ray variability of low-magnetic field neutron stars in low mass X-ray binaries, and that of black holes. In particular at frequencies <100 Hz, their power spectra can be strikingly similar. The highest frequency phenomena (kilohertz QPOs, black hole high-frequency QPOs and neutron star hectohertz QPOs) are the ones that showmost differences, perhaps because they originate closest to the compact object. Most variability components vary in frequency in correlation with one another, and the correlations once again are very similar across neutron stars and black holes — some extend even to white dwarfs. Although this does not strictly require that all phenomena whose frequencies are involved are caused by the same physics in all three source types, this does indicate that basic properties of the accretion flow which are the same in all three source types play an important role in generating at least some of the frequencies.

Relations between Timing Features and Colors in Accreting Millisecond Pulsars

We have studied the aperiodic X-ray timing and color behavior of the accreting millisecond pulsars SAX J1808.4-3658, XTE J1751-305, XTE J0929-314, and XTE J1814-338 using large data sets obtained with the Rossi X-ray Timing Explorer. We find that the accreting millisecond pulsars have very similar timing properties to the atoll sources and the low luminosity bursters. Based on the correlation of timing and color behavior SAX J1808.4-3658 can be classified as an atoll source, and XTE J0929-314 and XTE J1814-338 are consistent with being atoll sources, but the color behavior of XTE J1751-305 is different. Unlike in other atoll sources the hard color in the extreme island state of XTE J1751-305 is strongly correlated with soft color and intensity, and is not anti-correlated with any of the characteristic frequencies. We found previously, that the frequencies of the variability components of atoll sources follow a universal scheme of correlations. The frequency correlations of the accreting millisecond pulsars are similar, but in the case of SAX J1808.4-3658 and XTE J0929-314 shifted compared to those of the other atoll sources in a way that is most easily described as a shift in upper and lower kilohertz QPO frequency by a factor close to 1.5. Although, we note that the shift in lower kilohertz QPO frequency is based on only one observation for SAX J1808.4-3658. XTE J1751-305 and XTE J1814-338, as well as the low luminosity bursters show no or maybe small shifts.

The aperiodic timing behaviour of the accretion–driven millisecond pulsar SAX J1808.4–3658

We studied the aperiodic X-ray timing behaviour of the accreting millisecond pulsar SAX J1808.4–3658. The source was recently found [Nature 424 (2003) 44] to be the first accreting millisecond pulsar that shows the kilohertz quasi-periodic oscillations (kilohertz QPOs) that are found in many other X-ray binaries with accreting neutron stars. The high frequency of these signals reflects the short dynamical time scales in the region near the compact object where they originate. We find that in addition to the kilohertz QPOs SAX J1808.4–3658 shows several low frequency timing features, based on which the source can be classified as a so–called atoll source. The frequencies of the variability components of the atoll sources follow a universal scheme of correlations. The correlations in SAX J1808.4–3658 are similar but show a shift in upper kilohertz QPO frequency. This discrepancy is perhaps related to a stronger or differently configured magnetic field.

The Atoll Source States of 4U 1608−52

We have studied the atoll source 4U 1608-52 using a large data set obtained with the Rossi X-ray Timing Explorer. We find that the timing properties of 4U 1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09 and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a transient covering two orders of magnitude in luminosity. The frequencies of the variability components of these three sources follow a universal scheme when plotted versus the frequency of the upper kilohertz QPO, suggesting a very similar accretion flow configuration. If we plot the Z sources on this scheme only the lower kilohertz QPO and HBO follow identical relations. Using the mutual relations between the frequencies of the variability components we tested several models; the transition layer model, the sonic point beat frequency model, and the relativistic precession model. None of these models described the data satisfactory. Recently, it has been suggested that the atoll sources (among them 4U 1608-52) trace out similar three-branch patterns as the Z sources in the color-color diagram. We have studied the relation between the power spectral properties and the position of 4U 1608-52 in the color-color diagram and conclude that the timing behavior is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually, as Z sources do.

Global solution for a viscous accretion disc around a rotating compact object: pseudo-general-relativistic study

We study the solution for a viscous accretion disc around a rotating compact/central object having a hard surface, i.e. a neutron star, a strange star or any other highly gravitating object. We choose a pseudo-Newtonian approach to describe the relativistic accretion disc. For this purpose, a new pseudo-Newtonian potential is established that is applicable to describing the relativistic properties of a star and its disc. As we know, the Hartle–Thorne metric can describe the geometry of a star as well as the space–time outside it, so we use this metric to establish our potential. Our potential reproduces the marginally stable orbit exactly as does general relativity. It also reproduces the marginally bound orbit and specific mechanical energy at the marginally stable orbit with at most 4 and 10 per cent error, respectively. Using this potential, we study the global parameter space of the accretion disc. Thus we find the physical parameter regime for which a stable accretion disc can be formed around a gravitating object with a hard surface. We also study how the fluid properties get changed with different rotations of the central star. We show that, with a change in rotation of the central object, the valid disc parameter region changes dramatically. We also show the effect of viscosity on the fluid properties of the disc. Subsequently, we give a theoretical prediction of kilohertz QPO frequencies, at least for one of a pair, for a fast-rotating compact object such as 4U 1636-53.

Probing the X-ray Variability of X-ray Binaries

Kilohertz quasi-periodic oscillations (kHz QPOs) has been regarded as representing the Keplerian frequency at the inner disk edge in the neutron star X-ray binaries. The so-called “parallel tracks” on the plot of the kHz QPO frequency vs. X-ray flux in neutron star X-ray binaries, on the other hand, show the correlation between the kHz QPO frequency and the X-ray flux on time scales from hours to days. This is suspected as caused by the variations of the mass accretion rate through the accretion disk surrounding the neutron star. We show here that by comparing the correlation between the kHz QPO frequency and the X-ray count rate on a certain QPO time scale observed approximately simultaneous in the Fourier power spectra of the X-ray light curve, we have found evidences that the X-ray flux of millihertz QPOs in neutron star X-ray binaries is generated inside the inner disk edge if adopting that the kilohertz QPO frequency is an orbital frequency at the inner disk edge.

A Multi‐Lorentzian Timing Study of the Atoll Sources 4U 0614+09 and 4U 1728−34

We present the results of a multi-Lorentzian fit to the power spectra of two kilohertz QPO sources; 4U 0614+09 and 4U 1728-34. This work was triggered by recent results of a similar fit to the black-hole candidates (BHCs) GX 339-4 and Cyg X-1 by Nowak in 2000. We find that one to six Lorentzians are needed to fit the power spectra of our two sources. The use of exactly the same fit function reveals that the timing behaviour of 4U 0614+09 and 4U 1728-34 is almost identical at luminosities which are about a factor 5 different. As the characteristic frequency of the Lorentzians we use the frequency, nu_max, at which each component contributes most of its variance per log frequency as proposed by Belloni, Psaltis & van der Klis in 2001. When using nu_max instead of the centroid frequency of the Lorentzian, the recently discovered hectohertz Lorentzian is practically constant in frequency. We use our results to test the suggestions by, respectively, Psaltis Belloni and van der Klis in 1999 and Nowak in 2000 that the two Lorentzians describing the high-frequency end of the broad-band noise in BHCs in the low state can be identified with the kilohertz QPOs in the neutron star low mass X-ray binaries. We find, that when the two kilohertz QPOs are clearly present, the low-frequency part of the power spectrum is too complicated to draw immediate conclusions from the nature of the components detected in any one power spectrum. However, the relations we observe between the characteristic frequencies of the kilohertz QPOs and the band-limited noise, when compared to the corresponding relations in BHCs, hint towards the identification of the second-highest frequency Lorentzian in the BHCs with the lower kilohertz QPO. They do not confirm the identification of the highest-frequency Lorentzian with the upper kilohertz QPO.

A Compton Upscattering Model for Soft Lags in the Lower Kilohertz Quasi-periodic Oscillation in 4U 1608−52

An empirical Compton up-scattering model is described which reproduces both the fractional amplitude (RMS) vs. energy and the soft time lags in the 830 Hz QPO observed in 4U1608-52 on Mar. 3, 1996. A combination of two coherent variations in the coronal and soft photon temperatures (with their relative contributions determined by enforcing energy conservation) gives rise to the QPO's energy dependent characteristics. All input parameters to the model, save a characteristic plasma size and the fraction of Comptonized photons impinging on the soft photon source, are derived from the time-averaged photon energy spectrum of the same observation. Fits to the fractional RMS and phase lag data for this kilohertz QPO imply that the spatial extent of the plasma is in the range from 4 to 15 km.

Magnetic fields of neutron stars in low mass X-ray binaries

Abstract We report detailed radiation transport calculations which show that the shape of the 1–30 keV continuum spectrum of a weakly magnetic accreting neutron star depends on the strength of its magnetic field. We use this dependence to estimate the magnetic fields of several accreting neutron stars in LMXBs. We demonstrate that the magnetic field strengths estimated in this way correlate strongly with the properties of the 15–60 Hz magnetospheric QPOs and the kilohertz QPOs observed in many Z and atoll sources. We also find that sources currently accreting at high rates have higher magnetic field strengths than sources currently accreting at lower rates. This is an important constraint on models of the evolution of the magnetic field strengths of accreting neutron stars.

On the Magnetospheric Beat‐Frequency and Lense‐Thirring Interpretations of the Horizontal‐Branch Oscillation in the Z Sources

We study the horizontal branch oscillations (HBO) and the two simultaneous kilohertz QPOs comparing their properties in five Z sources with the predictions of the magnetospheric beat-frequency and Lense-Thirring precession models. We find that the variation of the HBO frequency with accretion rate predicted by the magnetospheric beat-frequency model for a purely dipolar stellar magnetic field and a radiation-pressure-dominated inner accretion disk agrees well with the observed variation. This model implies that the neutron stars in the Z sources are near magnetic spin equilibrium, that their magnetic field strengths are ~10^9-10^10 G, and that the critical fastness parameter for these sources is >0.8. If the frequency of the upper kilohertz QPO is an orbital frequency in the accretion disk, the magnetospheric beat-frequency model requires that a small fraction of the gas in the disk does not couple strongly to the stellar magnetic field at 3-4 stellar radii but instead drifts slowly inward in nearly circular orbits until it is within a few kilometers of the neutron star surface. The Lense-Thirring precession model is consistent with the observed magnitudes of the HBO frequencies only if the moments of inertia of the neutron stars in the Z sources are 4-5 times larger than the largest values predicted by realistic neutron-star equations of state. The trend of the correlation between the HBO and upper kHz QPO frequencies is not consistent with this model. We argue that the change in the slope of the correlation between the frequency of the horizontal branch oscillation and the frequency of the upper kilohertz QPO, when the latter is greater than 850 Hz, is directly related to the varying frequency separation of the kilohertz QPOs.

The Kilohertz QPO Frequency and Flux Decrease in Aquila X-1 and the Effect of Soft X-Ray Spectral Components

We report on a Rossi X-Ray Timing Explorer observation of Aquila X-l during its outburst in 1997 March in which, immediately following a type I burst, the broadband 2-10 keV flux decreased by about 10% and the kilohertz quasi-periodic oscillation (kHz QPO) frequency decreased from 813 +/- 3 to 776 +/- 4 Hz. This change in kHz QPO frequency is much larger than expected from a simple extrapolation of a frequency-flux correlation that was established in data before the burst. Meanwhile, a very low frequency noise component in the broadband fast Fourier transform power spectra, with a fractional rms amplitude of 1.2% before the burst, ceased to exist after the burst. All these changes were accompanied by a change in the energy spectral shape. If we characterize the energy spectra with a model composed of two blackbody (BB) components and a power-law component, almost all the decrease in flux was in the two BE components. We attribute the two BE components to the contributions from a region very near the neutron star, or even from the neutron star itself, and from the accretion disk, respectively.