On the high coherence of kHz quasi-periodic oscillations

We have carried out a systematic study of the properties of the kHz quasi-periodic oscillations (QPOs) observed in the X-ray emission of the neutron star low-mass X-ray binary 4U 1608-052, using archival data obtained with the Rossi X-ray Timing Explorer. We have investigated the quality factor, Q, of the oscillations (defined as the ratio, v/Deltav, of the frequency v of the QPO peak to its full width at half-maximum Deltav). In order to minimize the effect of long-term frequency drifts, power spectra were computed over the shortest times permitted by the data statistics. We show that the high Q of similar to200 reported previously for the lower-frequency kHz QPO is by no means exceptional, as we observe a mean Q value in excess of 150 in 14 out of the 21 observations analysed and Q can remain above 200 for thousands of seconds. The frequency of the QPO varies over the wide range 560-890 Hz and we find a systematic trend for the coherence time of the QPO, estimated as tau = Q/(pi v) = 1/(piDeltav), to increase with v, up to a maximum level at similar to800 Hz, beyond which it appears to decrease, at frequencies where the QPO weakens. There is a more complex relationship between tau and the QPO rms amplitude, in which positive and negative correlations can be found. A higher-frequency QPO, revealed by correcting for the frequency drift of the 560-890 Hz one, has a much lower Q (similar to10) which does not follow the same pattern. We discuss these results in the framework of competing QPO models and we show that those involving clumps orbiting within or above the accretion disc are ruled out.