On the incidence of KHz quasi - periodic oscillations in the neutron star system 4 U 1636 - 53

Nearly 1.32 × 10 6 sec of non-zero count rate data were obtained from five years of RXTE observations of the atoll source 4U 1636-53. The data was divided into 10309 segments of 128 sec 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 kHz QPO was undertaken and the histograms of the maximum Leahy normalised power P max 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 fit 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 c/s) to zero for the highest one (> 2550 c/s) while the average amplitude remains nearly constant. Based on the best fit probability distribution functions, a threshold P thre was defined, such that 95% of the segments with P max > P thre 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's flux level is intermediate between two flux states.