Kilohertz QPOs — the link with the spin

The possibility of millisecond neutron star spin periods in LMXBs has been discussed in earnest since the recycling scenario for the formation of millisecond radio pulsars was first proposed in the early 1980's. A beat frequency interpretation of the then newly discovered low-frequency QPOs in Z sources involving millisecond spins was proposed in 1985. After the discovery of the kHz QPOs in LMXBs with RXTE in 1996, the possibility of a link with the neutron star spin has been discussed extensively again. The discoveries of the accreting millisecond pulsars from 1998 onward and of kHz QPOs in several of these from 2003 gave further impetus to the debate about to what extent aperiodic phenomena in LMXBs have a direct link with the spin. I review the current status of this discussion. Beat frequency models that were originally proposed for the kHz QPOs are no longer viable. If a relation of the QPO frequencies with the spin exists, then this requires a rather specific, presumably resonant, interaction of the spin with the accretion flow forcing matter to move in step with the spin somewhere in the flow. The evidence for a link of the kHz QPOs with neutron star spin has not over the years become more convincing despite much more data, yet connections do seem to exist. I suggest that this provides a hint as to how the kHz and other QPO frequencies arise and what is the optimal approach to satisfactorily describing them as a physical phenomenon.