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 Ross i X-Ray Timing Explorer (RXTE). We find that when oscillations are pre sent during burst rise, their amplitudes systematically decrease as th e X-ray burst flux increases, reaching undetectable levels near the bu rst peaks before eventually increasing again later in the burst. Oscil lation 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 underlum inous compared with those at intervals later in the burst with similar measured blackbody temperatures; this argues that the thermonuclear b urning is nonuniform during burst rise and spreads to encompass the st ar in 0.5 s. Thus, rotational modulation of the expanding hot spot sho uld be expected, assuming that the viewing geometry is favorable. We p resent the results of theoretical calculations of the light curve from a rotating neutron star with such a growing hot spot and show that th is model can account for both the observed initial pulsation amplitude s and their subsequent decrease during the rising phase. We argue that these results provide strong evidence in favor of the rotational modu lation hypothesis for the millisecond oscillations seen during X-ray b ursts with RXTE.