Millisecond Phenomena in Accreting Neutron Stars -- an Update

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Since the initial discoveries with the Rossi X-ray Timing Explorer (RXTE) in 1996 of kilohertz quasi-periodic oscillations (kHz QPOs) and burst oscillations in a number of low-mass X-ray binaries (LMXBs) containing low-magnetic-field neutron stars, a very active field has developed. I briefly summarize some of the developments since those early days, which include the discovery of the first accreting millisecond pulsar, further claims of the detection of strong-field general-relativistic effects, intriguing correlations of the kHz QPO properties with QPOs at lower frequencies, strong challenges for the beat-frequency interpretation of the twin kHz QPO peaks and a number of new theoretical ideas, some of which involving frame dragging. Twenty LMXBs have now been seen to exhibit periodic or quasi-periodic phenomena with frequencies exceeding 10².5 Hz. Although the commensurabilities between the observed frequencies that suggest a beat-frequency interpretation have conclusively been shown to be not precise, the preponderance of the evidence still is in favour of the idea that there exists some kind of beat-frequency relation between the twin kHz peaks. However, that evidence is only coming from 4 of the 17 sources showing twin kHz QPOs.