2006MNRAS.371.1925M

I study the behaviour of the maximum rms fractional amplitude, r_max, and the maximum coherence, Q_max, of the kilohertz quasi-periodic oscillations (kHz QPOs) in a dozen low-mass X-ray binaries. I find that (i) the maximum rms amplitudes of the lower- and upper-kHz QPOs, r^ℓ_max and r^u_max, respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower-kHz QPO, Q^ℓ_max, first increases and then decreases exponentially with luminosity, at a faster rate than both r^ℓ_maxand r^u_max; (iii) the maximum coherence of the upper-kHz QPO, Q^u_max, is more or less independent of luminosity; and (iv) r_maxand Q_maxshow the opposite behaviour with hardness of the source, consistent with the fact that there is a general anticorrelation between luminosity and spectral hardness in these sources. Both r_maxand Q_max in the sample of sources, and the rms amplitude and coherence of the kHz QPOs in individual sources show a similar behaviour with hardness. This similarity argues against the interpretation that the drop of coherence and rms amplitude of the lower-kHz QPO at high QPO frequencies in individual sources is a signature of the innermost stable circular orbit around a neutron star. I discuss possible interpretations of these results in terms of the modulation mechanisms that may be responsible for the observed variability.