Relations between Timing Features and Colors in the X‐Ray Binary 4U 0614+09

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Abstract
We study the correlations between timing and X-ray spectral properties in the low-mass X-ray binary 4U 0614+09 using a large (265 ks) data set obtained with the Rossi X-Ray Timing Explorer. We find strong quasi-periodic oscillations (QPOs) of the X-ray flux, like the kilohertz QPOs in many other X-ray binaries with accreting neutron stars, with frequencies ranging from 1329 Hz down to 418 Hz and perhaps as low as 153 Hz. We report the highest frequency QPO yet from any low-mass X-ray binary at 1329 ± 4 Hz, which has implications for neutron star structure. This QPO has a 3.5 σ single-trial significance; for an estimated 40 trials the significance is 2.4 σ. Besides the kilohertz QPOs, the Fourier power spectra show four additional components: high-frequency noise (HFN), described by a broken power law with a break frequency between 0.7 and 45 Hz, very low frequency noise (VLFN), which is fitted as a power law below 1 Hz, and two broad Lorentzians with centroid frequencies varying from 6 to 38 Hz and from 97 to 158 Hz, respectively. We find strong correlations between the frequencies of the kilohertz QPOs, the frequency of the 6-38 Hz broad Lorentzian, the break frequency of the HFN, the strength of both the HFN and the VLFN, and the position of the source in the hard X-ray color versus intensity diagram. The frequency of the 97-158 Hz Lorentzian does not correlate with these parameters. We also find that the relation between power density and break frequency of the HFN is similar to that established for black hole candidates in the low state. We suggest that the changing mass accretion rate is responsible for the correlated changes in all these parameters.
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