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 in the Galactic bulge (d & 9 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 μs earlier than the soft X-ray pulses. The pulsar was spinning down at an average rate of ν̇ = (−9.2±0.4)×10 Hz s; 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 × 10M⊙) of any stellar binary. The binary parameters imply a ≃ 0.01M⊙ 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. Subject headings: binaries: close — pulsars: individual (XTE J0929−314) — stars: neutron — stars: low-mass, brown dwarfs — X-rays: binaries