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;