We have identified the third known accretion-powered millisecond pulsar , XTE J0929 - 314 , with the Rossi X-Ray Timing Explorer .
The source is a faint , high–Galactic-latitude X-ray transient ( d \gtrsim 5 kpc ) that was in outburst during 2002 April–June .
The 185 Hz ( 5.4 ms ) pulsation had a fractional rms amplitude of 3–7 % and was generally broad and sinusoidal , although occasionally double-peaked .
The hard X-ray pulses arrived up to 770 \mu s earlier than the soft X-ray pulses .
The pulsar was spinning down at an average rate of \dot { \nu } = ( -9.2 \pm 0.4 ) \times 10 ^ { -14 } Hz s ^ { -1 } ; the spin-down torque may arise from magnetic coupling to the accretion disk , a magnetohydrodynamic wind , or gravitational radiation from the rapidly spinning pulsar .
The pulsations were modulated by a 43.6 min ultracompact binary orbit , yielding the smallest measured mass function ( 2.7 \times 10 ^ { -7 } M _ { \odot } ) of any stellar binary .
The binary parameters imply a \simeq 0.01 M _ { \odot } white dwarf donor and a moderately high inclination .
We note that all three known accreting millisecond pulsars are X-ray transients in very close binaries with extremely low mass transfer rates .
This is an important clue to the physics governing whether or not persistent millisecond pulsations are detected in low-mass X-ray binaries .