Publication date: Dec 2007
We further investigate the issue of clustering of kHz QPO frequency ratios in neutron star low mass X-ray binaries. In this note we report on the recent analysis of occurrences and properties of kHz QPOs in the source 4U 1636-53. Assuming that kHz QPOs occur in pairs whose frequencies are linearly correlated, we find a prominent frequency (or a narrow frequency region) that separates upper and lower QPO observations. The two QPO modes are then simultaneously detected mainly in the vicinity of this transition points. We show that this can be understood in terms of correlations of QPO properties with frequency, such as quality factor and rms amplitude. We find that rms amplitudes and quality factors of both QPOs nearly equal at the transition point. In addition, the QPO frequencies are nearly commensurable there. We investigate also five other atoll sources obtaining similar results.
Török, Gabriel; Bursa, Michal; Horák, Jiří; Stuchlík, Zdeněk; Bakala, Pavel;