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 1608−52, using archival data obtained with theRossi X-ray Timing Explorer. We have investigated the quality factor,Q, of the oscillations (defined as the ratio,, of the frequencyν of the QPO peak to its full width at half-maximumΔν). 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 highQof∼200 reported previously for the lower-frequency kHz QPO is by no means exceptional, as we observe a meanQvalue in excess of 150 in 14 out of the 21 observations analysed andQcan 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, to increase withν, up to a maximum level at∼800 Hz, beyond which it appears to decrease, at frequencies where the QPO weakens. There is a more complex relationship betweenτ 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 lowerQ(∼10) 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. [ABSTRACT FROM AUTHOR]