While millisecond oscillations in bursts were discovered shortly after the launch of RXTE, it took more than 2 y until RXTE in April 1998 finally also found coherent ms pulsations at a frequency of 401 Hz in the persistent flux of the transient and bursting LMXB SAX J1808.4-3658  (a reanalysis of earlier BeppoSAX data revealed a marginal detection of a 401 Hz oscillation in a burst). During the third outburst of this source in October 2002 oscillations with 401 Hz were clearly seen in all four bursts observed , establishing that the burst oscillations trace the spin of the NS. In addition, twin kHz QPOs were discovered with Av equal to 200 Hz (half of the burst frequency) . Today, a group of six objects with millisecond coherent periodicity in their persistent flux are known. They are all transients. Two of them, SAX J1808.4-3658 and XTE J1814-338, show also millisecond burst oscillations.
These accreting ms X-ray pulsars provide the long sought link to the rotation powered ms radio pulsars . The respective magnetic fields of both classes are in the range of (1 - 10) 108 Gauss. For such fast spinning objects, a stronger magnetic field would prevent accretion. The conventional wisdom is that the ms radio pulsars have developed from NS in LMXB, which had been spun up over a long time by accretion - we now see the progenitors. For rotation powered X-ray pulsars see .
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