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We report the discovery of an episode of coherent millisecond X-ray pulsation in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for slightly more than 150 seconds, during which the pulse frequency is consistent with being constant. No X-ray burst or other evidence of thermonuclear burning activity is seen in correspondence with the pulsation, which can thus be identified as occurring in the persistent emission. The pulsation frequency is 550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from burst oscillations in this source. Hence we identify this frequency with the neutron star spin frequency. The pulsed fraction is strongly energy dependent, ranging from <1% at 3-5 keV to >10% at 16-30 keV. We discuss possible physical interpretations and their consequences for our understanding of the lack of pulsation in most neutron star low-mass X-ray binaries. If interpreted as accretion-powered pulsation, Aql X-1 might play a key role in understanding the differences between pulsating and non-pulsating sources.

We present an analysis of the high frequency timing properties of the April-May 2000 outburst of the black hole candidate and Galactic microquasar XTE J1550−564, measured with the Rossi X-ray Timing Explorer. The rapid X-ray variability we measure is consistent with the source being in either the "very high" or "intermediate" black hole state. Strong (5-8% rms) quasi-periodic oscillations (QPOs) are found between 249-276 Hz; this represents the first detection of the same high frequency QPO in subsequent outbursts of a transient black hole candidate. We also present evidence for lower-frequency QPOs at approximately 188 Hz (3.5σ, single trial), also reported previously and likely present simultaneously with the higher-frequency QPOs. We discuss these findings within the context of the 1998 outburst of XTE J1550−564, and comment on implications for models of QPOs, accretion flows, and black hole spin.

We have detected an x-ray nebula around the binary millisecond pulsar B1957+20.A narrow tail, corresponding to the shocked pulsar wind, is seen interior to the known Hα bow shock and proves the long-held assumption that the rotational energy of millisecond pulsars is dissipated through relativistic winds. Unresolved x-ray emission likely represents the shock where the winds of the pulsar and its companion collide. This emission indicates that the efficiency with which relativistic particles are accelerated in the post shock flow is similar to that for young pulsars, despite the shock proximity and much weaker surface magnetic field of this millisecond pulsar.Millisecond pulsars are old neutron stars (typically ∼ 3 Gyr) that have been spun up to a rapid rotation rate ( < ∼ 25 ms) by accretion of material from a binary companion (1, 2). After the 1