Publication date: Dec 2007
Abstract:
Several models have been outlined to explain the (upper and lower) kilohertz quasi-periodic oscillations (QPOs) detected in many accreting neutron star X-ray binaries. When facing the theory to observation, rather limited attention has been payed to the mutual relations between the (correlated) QPO amplitudes and quality factors till now. In this paper we report on recent results on these relations. For six neutron star atoll sources (namely 4U 1728-34, 4U 1608-52, 4U 1636-53, 4U 0614+09, 4U 1820-30 and 4U 1735-44) spanning wide range of frequencies we investigate whether the relationship between the rms amplitudes and quality factors of the observed kHz QPO modes ν _{L}, ν _{U} display features that could have a significant meaning in terms of the proposed QPO models. We find for all the six sources that after the twin kHz QPOs pass a point (or the narrow interval) where their ratio R equals 1.5 the lower/upper oscillation becomes stronger/weaker than other one with increasing QPO frequency. Existence of a similar effect close to R = 1.33 or R = 1.25 is also indicated. Moreover, for increasing QPO frequency, shortly after passing 3/2 ratio, the difference between QPO amplitudes as well as lower QPO quality factor reaches its maxima on a narrow frequency interval where lower QPO is much stronger than the upper one. This interval lies between frequencies corresponding to 3/2 and 4/3 (or 5/4) frequency ratio. This finding implies restrictions to the orbital QPO models (both hot spot- and disc oscillations- like) and also to QPO modulation mechanism. In a wider context, our results may indicate the existence of an energy overflow between the upper and lower QPO mode when their ratio is close to ratio of small integral numbers.
Authors:
Török, Gabriel; Stuchlík, Zdeněk; Čermák, Petr;