MILLISECOND X-RAY VARIABILITY FROM AN ACCRETING NEUTRON-STAR SYSTEM

We report the detection with the Proportional Counter Array (PCA) on b oard the Rossi X-Ray Timing Explorer (RXTE) of millisecond variability in the X-ray emission from the low-mass X-ray binary 4U 1728-34. Puls ations at 363 Hz with amplitudes (rms) of 2.5%-10% are present in six of the eight bursts analyzed to date. The strongest were seen in two s uccessive bursts recorded on 1996 February 16 when the quiescent count rate was near the highest seen by PCA. The pulsations during these bu rsts show frequency changes of 1.5 fit during the first few seconds bu t become effectively coherent as the burst decays. We interpret the 36 3 Hz pulsations as rotationally induced modulations of inhomogeneous b urst emission. This represents the first compelling evidence for a mil lisecond spin period in a low-mass X-ray binary. Complex, intensity-de pendent, millisecond X-ray variability is also present in all the quie scent flux intervals we examined. Most interesting was the behavior as the count rate approached its highest observed level. Two quasi-perio dic oscillations (QPOs) were simultaneously observed in the 650-1100 H z range. Both QPOs increased in frequency together, maintaining a near ly constant frequency separation of about 363 Hz, the spin period infe rred from the burst oscillations. This phenomenology is strongly sugge stive of the magnetospheric beat frequency model proposed for the hori zontal-branch oscillations (HBOs) seen in Z sources. We discuss this a nd several other possible physical interpretations for the observed X- ray variability.