A millisecond pulsar in an X-ray binary system.

Ordinary radio pulsars' are neutron stars with magnetic fields of similar to 10(12) gauss and spin periods in the range 0.1 to 3 seconds. In contrast, millisecond radio pulsars(2) have much weaker fields (similar to 10(9) gauss) and faster, millisecond spire rates. For both types of pulsar, the energy driving the radio pulsations is thought to be derived from the rotation of the neutron star. The star gradually 'spins down' as energy is radiated away. Millisecond radio pulsars are often located in binary systems(3). In a widely accepted theoretical model(4,5), they started as ordinary pulsars which lose most of their magnetic field and were 'spun up' to millisecond periods by the accretion of matter from a companion star in an X-ray binary system. Evidence(6-11) for this model has gradually mounted, but direct proof-in the form of the predicted coherent millisecond X-ray pulsations in the persistent flux of are X-ray binary has been lacking, despite many searches(12-15). Here we report the discovery(16) of such a pulsar, confirming theoretical expectations. The source will probably become a millisecond radio pulsar when the accretion turns off completely.