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

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Abstract
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.
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