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 thermonuclear 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 exhibit both millisecond oscillations during the rising phase and photospheric radius expansion. We measure an oscillation amplitude within 0.1 s of the onset of this burst of 75% +/- 17%; that is, almost the entire thermal burst flux is modulated near onset. The spectral evolution during the rising phase of this burst suggests that the X-ray-emitting area on the neutron star was increasing, similar to the behavior of the bursts from 4U 1728-34 with 363 Hz oscillations reported recently. We argue that the combination of large pulsed amplitudes near burst onset and the spectral evidence for localized emission during the rise strongly supports rotational modulation as the mechanism for the oscillations. We discuss how a theoretical interpretation of spin modulation amplitudes, pulse profiles, and pulse-phase spectroscopy can provide constraints on the masses and radii of neutron stars. We also discuss the implications of these findings for the beat-frequency models of kilohertz X-ray variability in LMXBs.