kHz quasi-periodic oscillations in the low-mass X-ray binary 4U 0614+09

We report on a comprehensive analysis of the kilohertz (≥300 Hz) quasi-periodic oscillations (kHz QPOs) detected from the neutron star low-mass X-ray binary 4U 0614+09 with the Rossi X-ray Timing Explorer. With a much larger data set than previously analysed (all archival data from 1996 February up to 2007 October), we first investigate the reality of the 1330 Hz QPO reported by van-Straaten et al. This QPO would be of particular interest since it has the highest frequency reported for any source. A thorough analysis of the same observation fails to confirm the detection. On the other hand, over our extended data set, the highest QPO frequency we measure for the upper kHz QPO is at ∼1224 Hz; a value which is fully consistent with the maximum values observed in similar systems. Secondly, we demonstrate that the frequency dependence of the quality factor  ( Q =ν/Δν)  and amplitude of the lower and upper kHz QPOs follow the systematic trends seen in similar systems. In particular, 4U 0614+09 shows a drop of the quality factor of the lower kHz QPO above ∼700 Hz. If this is due to an approach to the innermost stable circular orbit, it implies a neutron star mass of  ∼1.9 M_⊙  . Finally, when analysing the data over fixed durations, we have found a gap in the frequency distribution of the upper QPO, associated with a local minimum of its amplitude. A similar gap is not present in the distribution of the lower QPO frequencies, suggesting some cautions when interpreting frequency ratio distributions, based on the occurrence of the lower QPO only.     

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  ν₁∼ν ^b ₂  between the upper and the lower kHz QPOs with different indices: b ≃ 1.5 for the Atoll source 4U 1728-34 and b ≃ 1.9 for the Z source Sco X-1. The implications of our results for the theoretical models for kHz QPOs are discussed.     

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.     

Rossi XTE monitoring of 4U 1636–53 – I. Long-term evolution and kHz quasi-periodic oscillations

We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ) for more than 1.5 yr. Our campaign consisted of short (∼2 ks) pointings separated by 2 d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ∼30–40 d, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kilohertz (kHz) quasi-periodic oscillations (QPOs) in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency.     

Kilohertz quasi-periodic oscillations difference frequency exceeds inferred spin frequency in 4U 1636−53

We report the first detection of a sharp spectral feature in a narrow-line Seyfert 1 galaxy. Using XMM-Newton we have observed 1H     and find a drop in flux by a factor of more than 2 at a rest-frame energy of ∼ 7 keV without any detectable narrow Fe K α line emission. The energy of this feature suggests a connection with the neutral iron K photoelectric edge, but the lack of any obvious absorption in the spectrum at lower energies makes the interpretation challenging. We explore two alternative explanations for this unusual spectral feature: (i) partial-covering absorption by clouds of neutral material; and (ii) ionized disc reflection with lines and edges from different ionization stages of iron blurred together by relativistic effects. We note that both models require an iron overabundance to explain the depth of the feature. The X-ray light curve shows strong and rapid variability, changing by a factor of 4 during the observation. The source displays modest spectral variability which is uncorrelated with flux.     

Evidence of a decrease of kHz quasi-periodic oscillation peak separation towards low frequencies in 4U 1728–34 (GX 354–0)

We have produced the colour–colour diagram of all the observations of 4U 1728–34 available in the Rossi X-ray Timing Explorer public archive (from 1996 to 2002) and found observations filling in a previously reported 'gap' between the island and the banana X-ray states. We have made timing analysis of these gap observations and found, in one observation, two simultaneous kHz quasi-periodic oscillations (QPOs). The timing parameters of these kHz QPOs fit in the overall trend of the source. The 'lower' kHz QPO has a centroid frequency of ∼308 Hz. This is the lowest 'lower' kHz QPO frequency ever observed in 4U 1728–34. The peak frequency separation between the 'upper' and the 'lower' kHz QPO is  Δν= 274 ± 11 Hz  , significantly smaller than the constant value of  Δν∼ 350 Hz  found when the 'lower' kHz QPO frequency is between ∼500 and 800 Hz. This is the first indication in this source for a significant decrease of kHz QPO peak separation towards low frequencies. We compare the result briefly to theoretical models for kHz QPO production.     

The coherence of kilohertz quasi-periodic oscillations in the X-rays from accreting neutron stars

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.     

The harmonic and sideband structure of the kilohertz quasi-periodic oscillations in Sco X-1

We use data from the Rossi X-ray Timing Explorer to search for harmonics and sidebands of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in Sco X-1. We do not detect any of these harmonics or sidebands, with 95 per cent confidence upper limits to their power between ∼1 and ∼10 per cent of the power of the upper kHz QPO. The oscillations produced at these frequencies may be attenuated in a scattering corona around the neutron star. We find that upper limits to the unattenuated power of some of the strongest theoretically predicted harmonics and sidebands are as low as ∼2 per cent of the unattenuated power of the high-frequency QPO in Sco X-1.     

The frequencies of high frequency quasi-periodic oscillations in two different frames in black hole LMXBs

We discuss the modes of the Alfve'n waves in the accretion disk with a toroidal magnetic field in black hole low mass X-ray binaries in a rotating frame.By solving the perturbed general relativistic magnetohydrodynamic equations in the rotating frame,we find two stable modes of the Alfve'n wave which are the same as those in the fiducial observer frame.This gives a feasible way to transform between the two different frames,which validates the possible Alfve'n wave modes in the accreting celestial bodies wit...     

Observations of pulsating X-ray binaries 4U0900-40 and 4U1223-62

A rocket observation of two pulsating X-ray binaries, 4U0900-40 and 4U1223-62, found no evidence for periodic or quasiperiodic behavior on time scales down to 2 ms. Evidence for complex high-energy spectral features and intrapulse variability is presented for 4U0900-40.     

The host galaxy of a narrow-line Seyfert 1 galaxy, RE J1034+396, with X-ray quasi-periodic oscillations

Using simple stellar population synthesis, we model the bulge stellar contribution in the optical spectrum of a narrow-line Seyfert 1 galaxy, RE J1034+396. We find that its bulge stellar velocity dispersion is  67.7 ± 8 km s⁻¹  . The supermassive black hole (SMBH) mass is about  (1–4) × 10⁶ M_⊙  if it follows the well-known   M _BH–σ_*  relation found in quiescent galaxies. We also derive the SMBH mass from the Hβ second moment, which is consistent with that from its bulge stellar velocity dispersion. The SMBH mass of (1–4)  × 10⁶ M_⊙  implies that the X-ray quasi-periodic oscillation (QPO) of RE J1034+396 can be scaled to a high-frequency QPO at 27–108 Hz found in Galactic black hole binaries with a  10-M_⊙  black hole. With the mass distribution in different age stellar populations, we find that the mean specific star formation rate (SSFR) over the past 0.1 Gyr is  0.0163 ± 0.0011  Gyr⁻¹, the stellar mass in the logarithm is  10.155 ± 0.06  in units of solar mass and the current star formation rate is  0.23 ± 0.016 M_⊙ yr⁻¹  . For RE J1034+396, there is no relation between the Eddington ratio and the SSFR as suggested by Chen et al., despite a larger scatter in their relation. We also suggest that about 7.0 per cent of the total Hα luminosity and 50 per cent of the total [O  ii ] luminosity come from the star formation process.     

The applications of the MHD Alfvén wave oscillation model for kHz quasi‐periodic oscillations

In this paper, we improve the previous work on the MHD Alfvén wave oscillation model for the neutron star (NS) kHz quasi‐periodic oscillations (QPOs), and compare the model with the updated twin kHz QPO data. For the 17 NS X‐ray sources with the simultaneously detected twin kHz QPO frequencies, the stellar mass M and radius R constraints are given by means of the derived parameter A in the model, which is associated with the averaged mass density of the star as 〈ρ 〉 = 3M /(4πR³) ≃ 2.4 × 10¹⁴ (A /0.7)² g/cm³, and we also compare the M ‐R constraints with the stellar equations of state. Moreover, we also discuss the theoretical maximum kHz QPO frequency and maximum twin peak separation, and some expectations on SAX J1808.4–3658 are mentioned, such as its highest kHz QPO frequency ∼ 870 Hz, which is about 1.4–1.5 times less than those of the other known kHz QPO sources. The estimated magnetic fields for both Z sources (about Eddington accretion rate ) and Atoll sources (∼ 1% ) are approximately ∼10⁹ G and ∼10⁸ G, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)