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

Abstract

Nearly 1.32 × 10 6 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 10 309 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 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-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 P thre was defined, such that 95 per cent 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 flux level is intermediate between two flux states.

Parallel Tracks as Quasi-steady States for the Magnetic Boundary Layers in Neutron-star Low-mass X-Ray Binaries

“…Note that our restriction for generating tracks in the high-Ṁ regime with relatively high values of δ results in an effective suppression of QPOs as if there is a thresholdṀ at sufficiently high flux levels. Such a behavior has been observed before in the atoll sources 4U1608-52, 4U1636-53, and AqlX-1, where the n kHz range of high-intensity tracks is narrower than that of lowintensity tracks Misra & Shanthi 2004;Barret et al 2008).…”

“…The observed shifts in the atoll track were proposed by Di Salvo et al (2003) as a possible reason behind the lack of a strong correlation between the QPO frequencies and the color-color position of the source over the long term. To quantify the incidence of kHz QPOs in 4U1636-53, Misra & Shanthi (2004) used the five-year data of the source to obtain the histogram of the number of detections of QPOs as a function of count rate. The incidence of kHz QPOs was found to decrease as the flux level increases over the long term.…”

“…Besides the spectral shape, the long-term intensity may also be crucial in understanding the properties of kHz QPOs (Misra & Shanthi 2004). The possible existence of an inferred thresholdṀ, beyond which QPOs seem to be suppressed, can be revealed by the long-term intensity evolution of the source.…”

“…The possible existence of an inferred thresholdṀ, beyond which QPOs seem to be suppressed, can be revealed by the long-term intensity evolution of the source. If there is a thresholdṀ in the long-term flux evolution of the source as suggested by the work of Misra & Shanthi (2004), the flux can then be a good indicator ofṀ not only in the short term but also over the long term. The lack of correlation between n kHz and F X over the long term can be due to another parameter or parameters determining n kHz in addition toṀ.…”

“…The model can yield disconnected tracks if the observational windowing is simulated by gaps between numerical data sets. If, however, the distinct tracks correspond to certain flux levels at which the kHz QPOs can be produced, the gaps may not only arise from undersampling (Misra & Shanthi 2004). The direct observation of several transitions between the two parallel tracks of 4U1636-53 by Méndez (2003) could be the first evidence that kHz QPOs can only be generated at some definite flux levels.…”