Frequencies Of Kilohertz Quasi-periodic Oscillations Research Articles

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.

On the correlated spectral and timing properties of 4U 1636–53: An atoll source at high accretion rates

We analyze ~600 ks data from the X-ray observatory Rossi X-ray Timing Explorer of the neutron star low mass X-ray binary and atoll source 4U 1636–53. These observations span almost three years, from April 1996 to February 1999. The color-color and hardness-intensity diagrams show significant secular shifts of the atoll track, similar to what is observed for some Z sources. These are most evident in the hardness-intensity diagram, where shifts in intensity up to ~20% are observed. We find that the intensity shifts in the hardness-intensity diagram are responsible for the parallel tracks observed in the kilohertz quasi periodic oscillations frequency vs. intensity diagram. While the parallel tracks in the frequency vs. color plane partially overlap, systematic long term shifts are still evident. We also study the broad band power spectra of 4U 1636–53 as a function of the source position Sa along the track in the color-color diagram. These power spectra are all characteristic of the lower and upper banana state in atoll sources, showing very low frequency noise, band limited noise, and sometimes one or two kHz QPOs. We find that the very low frequency noise in some intervals is not well described by a power law because of an excess of power between 20 and 30 mHz, which can be fitted by a Lorentzian. Also, the characteristic frequency of the band limited noise shows a trend to decrease with increasing Sa at high Sa values.

Kilohertz Quasi-periodic Oscillation Frequency Anticorrelated with Megahertz Quasi-periodic Oscillation Flux in 4U 1608-52

We analyzed Rossi X-Ray Timing Explorer data of the low-mass X-ray binary and atoll source 4U 1608-52 obtained on 1996 March 3 in which the source simultaneously showed a strong single kilohertz quasi-periodic oscillation (kHz QPO) around 840 Hz and a 7.5 mHz QPO detected at energies below 5 keV. We find that the frequency of the kHz QPO is approximately anticorrelated with the 2-5 keV X-ray count rate associated with the mHz QPO. The average kHz QPO frequency varies by about 0.6 Hz (0.07%) during a mHz QPO cycle over which the average 2-5 keV count rate varies by about 60 counts s(-1) (4%). This is the opposite of the frequency-count rate correlation observed in the same data on longer timescales and, hence, constitutes the first example of a sign reversal in the frequency-flux correlation related to the origin of the flux. Such a sign reversal is predicted by the radiative disk truncation model for the case where the flux variations originate on the neutron star but are not due to disk accretion rate fluctuations. The results support the nuclear burning interpretation of the mHz QPO and the interpretation of the kHz QPO frequency as an indicator of the orbital frequency at the inner edge of the accretion disk. The varying radiative stresses exerted on the inner disk by the flux due to the quasi-periodic nuclear burning lead to changes in the inner disk radius and, hence, to the observed anticorrelation between kHz QPO frequency and X-ray count rate. We found a time lag of about 10 s of the X-ray count rate relative to the kHz QPO frequency in terms of the anticorrelation between these two quantities, which could be caused by the propagation of the nuclear burning front on the neutron star away from the equatorial region.

Correlation between the kilohertz quasi-periodic oscillation and energy spectrum of Aql X-1

From an analysis of the observed data of the X-ray transient Aquila X-1 on 1997 February 27 and March 1, we find that there exists some correlation between the frequency of kilohertz quasi-periodic oscillations and the shape of X-ray spectrum, similar to the case of the X-ray transient 4U 1608-52. Implications of our results for the accretion and QPO production process are discussed.

Correlations between Kilohertz Quasi-periodic Oscillations and Low-Frequency Features Attributed to Radial Oscillations and Diffusive Propagation in the Viscous Boundary Layer around a Neutron Star

We present a dimensional analysis of two characteristic timescales in the boundary layer where the disk adjusts to the rotating neutron star (NS). The boundary layer is treated as a transition region between the NS surface and the first Keplerian orbit. The radial transport of the angular momentum in this layer is controlled by a viscous force defined by the Reynolds number, which in turn is related to the mass accretion rate. We show that the observed low-Lorentzian frequency is associated with radial oscillations in the boundary layer, where the observed break frequency is determined by the characteristic diffusion time of the inward motion of the matter in the accretion flow. Predictions of our model regarding relations between those two frequencies and the frequencies of kilohertz quasi-periodic oscillations (kHz QPOs) compare favorably with recent observations of the source 4U 1728-34. This Letter contains a theoretical classification of kHz QPOs in NS binaries and the related low-frequency features. Thus, results concerning the relationship between the low-Lorentzian frequency of viscous oscillations and the break frequency are presented in the framework of our model of kHz QPOs viewed as Keplerian oscillations in a rotating frame of reference.

Strong Correlation between Noise Features at Low Frequency and the KilohertzQuasi-Periodic Oscillations in the X-Ray Binary 4U 1728−34

We study the timing properties of the low-mass X-ray binary 4U 1728-34 using recently released data from the Rossi X-Ray Timing Explorer. This binary, like many others with accreting neutron stars, is known to exhibit strong quasi-periodic oscillations (QPOs) of its X-ray flux near 1 kHz. In addition to the kilohertz QPOs, the Fourier power spectra show a broken power-law noise component, with a break frequency between 1 and50 Hz, and a Lorentzian between 10 and 50 Hz. We find that the frequencies of the break and the low-frequency Lorentzian are well correlated with the frequencies of the kilohertz QPOs. The correlation between the frequency of the low-frequency Lorentzian and the kilohertz QPO follows a power-law relation with an index of 2.11±0.06. The recent model of Stella & Vietri interprets the Lorentzian feature as an effect of relativistic frame dragging (Lense-Thirring precession) in the inner accretion disk. The predicted scaling index is 2, which is close to the value we measure. This correlation is nearly identical to the one found in Z-sources between the well-known QPOs on the horizontal branch and the kilohertz QPOs, suggesting that the low-frequency oscillations are a similar phenomenon in these sources. The frequency of the break in the power spectra is also correlated with the frequencies of the kilohertz QPOs. As previously noted for the similar binaries 4U 1608-50 and4U 1705-44, this broken power-law component closely resembles that of black hole candidates in the low state, where the break frequency is taken as a indicator of mass accretion rate. The relation between break frequency and kilohertz QPO frequency thus provides additional proof that the frequency of the kilohertz QPOs increases with mass accretion rate.