ON THE AMPLITUDE OF BURST OSCILLATIONS IN 4U-1636-54 - EVIDENCE FOR NUCLEAR-POWERED PULSARS

We present a study of 581 Hz oscillations observed during a thermonucl ear X-ray burst from the low-mass X-ray binary (LMXB) 4U 1636-54 with the Rossi X-Ray Timing Explorer. This is the first X-ray burst to exhi bit both millisecond oscillations during the rising phase and photosph eric radius expansion. We measure an oscillation amplitude within 0.1 s of the onset of this burst of 75% +/- 17%; that is, almost the entir e thermal burst flux is modulated near onset. The spectral evolution d uring the rising phase of this burst suggests that the X-ray-emitting area on the neutron star was increasing, similar to the behavior of th e bursts from 4U 1728-34 with 363 Hz oscillations reported recently. W e argue that the combination of large pulsed amplitudes near burst ons et and the spectral evidence for localized emission during the rise st rongly supports rotational modulation as the mechanism for the oscilla tions. We discuss how a theoretical interpretation of spin modulation amplitudes, pulse profiles, and pulse-phase spectroscopy can provide c onstraints on the masses and radii of neutron stars. We also discuss t he implications of these findings for the beat-frequency models of kil ohertz X-ray variability in LMXBs.