Discovery of a high-latitude accreting millisecond pulsar in an ultracompact binary

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 (dgreater than or similar to5 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 mus earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of (v)over dot =(9.2+/-0.4) x 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 minute ultracompact binary orbit, yielding the smallest measured mass function (2.7x10(-7) M-circle dot) of any stellar binary. The binary parameters imply an similar or equal to0.01 M-circle dot 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.