On the maximum amplitude and coherence of the kilohertz quasi-periodic oscillations in low-mass X-ray binaries

I study the behaviour of the maximum rms fractional amplitude, $r_{rm max}$ and the maximum coherence, $Q_{rm max}$, 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 the upper kHz QPO, $r^{ell}_{rm max}$ and $r^{rm u}_{rm max}$, respectively, decrease more or less exponentially with increasing luminosity of the source; (ii) the maximum coherence of the lower kHz QPO, $Q^{ell}_{rm max}$, first increases and then decreases exponentially with luminosity, at a faster rate than both $r^{ell}_{rm max}$ and $r^{rm u}_{rm max}$; (iii) the maximum coherence of the upper kHz QPO, $Q^{rm u}_{rm max}$, is more or less independent of luminosity; and (iv) $r_{rm max}$ and $Q_{rm max}$ show 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 $r_{rm max}$ and $Q_{rm max}$ 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.