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| - On the incidence of kilohertz quasi-periodic oscillations in the neutron star system 4U 1636−53
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| - Abstract. Nearly 1.32 × 106 s of non-zero count rate data were obtained from five years of Rossi X-ray Timing Explorer (RXTE) observations of the atoll source 4U 1636−53. The data were divided into 10309 segments of 128 s each. The histogram of the number of segments as a function of count rate shows that the system can naturally be classified into four flux states. For each segment, an automated search for kilohertz quasi-periodic oscillations (QPOs) was undertaken and the histograms of the maximum Leahy normalized power Pmax for the four flux states were created. These were fitted by probability distribution functions under the assumption that the intrinsic amplitude of the QPO has a normal distribution. The best-fitting distribution functions showed that the fraction f of the time the system exhibits a kHz QPO decreases from near unity for the lowest flux state (≈1400 count s−1) to zero for the highest (>2550 count s−1), whereas the average amplitude remains nearly constant. Based on the best-fitting probability distribution functions, a threshold Pthre was defined, such that 95 per cent of the segments with Pmax> Pthre correspond to a real QPO signal. For segments selected by this criterion, the frequency versus count rate plot reproduced the earlier known ‘parallel track’ variation with the tracks coinciding roughly with the flux states. It is shown that the gaps between the tracks are not caused by uneven sampling, but rather the QPO phenomenon is absent or weak when the system flux level is intermediate between two flux states.
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