Publication date: Mar 2008
Abstract:
A recently published study on long term evolution of the frequencies of
the kilohertz quasi-periodic oscillations (QPOs) in the atoll source 4U
1636-53 concluded that there is no preferred frequency ratio in a
distribution of twin QPOs that was inferred from the distribution of a
single frequency alone. However, we find that the distribution of the
ratio of actually observed pairs of kHz QPO frequencies is peaked close
to the 3/2 value, and possibly also close to the 5/4 ratio. To resolve
the apparent contradiction between the two studies, we examine in detail
the frequency distributions of the lower kHz QPO and the upper kHz QPO
detected in our data set. We demonstrate that for each of the two kHz
QPOs (the lower or the upper), the frequency distribution in all
detections of a QPO differs from the distribution of frequency of the
same QPO in the subset of observations where both the kHz QPOs are
detected. We conclude that detections of individual QPOs alone should
not be used for calculation of the distribution of the frequency ratios.
Authors:
Török, G.; Abramowicz, M. A.; Bakala, P.; Bursa, M.; Horák, J.; Kluzniak, W.; Rebusco, P.; Stuchlik, Z.;