Time- and Energy-dependent Characteristics of Thermonuclear Burst Oscillations

We have investigated temporal and spectral properties of a large sample of thermonuclear bursts with oscillations from eight different sources with spin frequencies varying from 270 to 620 Hz. For our sample, we chose those bursts for which the oscillation is sufficiently strong and of relatively long duration. The emission from the hot-spot that is formed during a thermonuclear burst is modulated by several physical processes and the burst oscillation profiles unavoidably carry signatures of these. In order to probe these mechanisms, we examined the amplitude and phase lags of the burst oscillations with energy. We also studied the frequency variation of oscillations during these thermonuclear bursts. We observed that the frequency drifts are more frequent in the cases where the spin frequency is lower. We found that the phase lags of the burst oscillations shows no systematic evolution with energy between the bursts, and also in between different sources. In seven cases, we do indeed observe lag of soft energy photons, while there is a significant number of cases for which hard lag or no lag is observed.