363 Hz oscillations during the rising phase of bursts from 4U 1728-34: Evidence for rotational modulation

We report a study of the temporal and spectral characteristics of the rising portions of X-ray bursts from 4U 1728-34 observed with the Rossi X-Ray Timing Explorer (RXTE). We find that when oscillations are present during burst rise, their amplitudes systematically decrease as the X-ray burst flux increases, reaching undetectable levels near the burst peaks before eventually increasing again later in the burst. Oscillation amplitudes as high as 43% +/- 12% are measured within the first 0.2 s of burst onset and have pulse shapes consistent with sinusoids. Spectral analysis indicates that the rises are substantially underluminous compared with those at intervals later in the burst with similar measured blackbody temperatures; this argues that the thermonuclear burning is nonuniform during burst rise and spreads to encompass the star in 0.5 s. Thus, rotational modulation of the expanding hot spot should be expected, assuming that the viewing geometry is favorable. We present the results of theoretical calculations of the light curve from a rotating neutron star with such a growing hot spot and show that this model can account for both the observed initial pulsation amplitudes and their subsequent decrease during the rising phase. We argue that these results provide strong evidence in favor of the rotational modulation hypothesis for the millisecond oscillations seen during X-ray bursts with RXTE.