SIMBAD references

Using archival data from the Rossi X-ray Timing Explorer (RXTE), we study in a systematic way the variation of the quality factor (Q=ν/Δν, Δν, 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 ∼550 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 ∼60 Hz of half the neutron star spin frequency (at ν_spin=581Hz). The quality factor of the lower kHz QPO increases with frequency up to a maximum of Q~ 200 at ν_lower~850Hz, then drops precipitously to Q~ 50 at the highest detected frequencies ν_lower~920Hz. 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∼ 5 to ∼15 all the way to ν_upper~1150Hz, 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/πΔν∼ 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 ∼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.