An abrupt drop in the coherence of the lower kHz quasi-periodic oscillations in 4U 1636-536

Using archival data from the Rossi X-ray Timing Explorer (RXTE), we study in a systematic way the variation of the quality factor (Q = v/Delta v, Delta v, full width at half maximum) and amplitude of the lower and upper kHz quasi-periodic oscillations (QPOs) in the low-mass X-ray binary 4U 1636-536, over a frequency range from similar to 550 to similar to 1200 Hz. When represented in a quality factor versus frequency diagram, the upper and lower QPOs follow two different tracks, suggesting that they are distinct phenomena, although not completely independent because the frequency difference of the two QPOs, when detected simultaneously, remains within similar to 60 Hz of half the neutron star spin frequency (at v(spin) = 581 Hz). The quality factor of the lower kHz QPO increases with frequency up to a maximum of Q approximate to 200 at v(lower) approximate to 850 Hz, then drops precipitously to Q approximate to 50 at the highest detected frequencies v(lower) approximate to 920 Hz. A ceiling of the lower QPO frequencies at 920 Hz is also clearly seen in a frequency versus count rate diagram. At the same time, the quality factor of the upper kHz QPO increases steadily from Q similar to 5 to similar to 15 all the way to v upper 1150 Hz, which is the highest detectable QPO frequency. The rms amplitudes of both the upper and lower kHz QPOs decrease steadily towards higher frequencies. The quality factor provides a measure of the coherence of the underlying oscillator. For exponentially damped sinusoidal shots, the highest Q observed corresponds to an oscillator coherence time of 1/pi Delta v similar to 0.1 s. All existing QPO models face challenges in explaining such a long coherence time and the significantly different behaviours of the quality factors of the upper and lower QPOs reported here. It is therefore difficult to be certain of the implications of the abrupt change in the lower QPO at similar to 850 Hz. We discuss various possible causes, including that the drop in coherence is ultimately caused by effects related to the innermost stable circular orbit.