A characterization of the brightness oscillations during thermonuclear bursts from 4U 1636-536

The discovery of nearly coherent brightness oscillations during thermonuclear X-ray bursts from six neutron star low-mass X-ray binaries has opened up a new way to study the propagation of thermonuclear burning and may ultimately lead to greater understanding of thermonuclear propagation in other astrophysical contexts, such as in Type Ia supernovae. Here, we report detailed analyses of the similar to 580 Hz brightness oscillations during bursts from 4U 1636-536. We investigate the bursts as a whole and, in more detail, the initial portions of the bursts. We analyze the similar to 580 Hz oscillations in the initial 0.75 s of the five bursts that were used in a previous search for a brightness oscillation at the expected similar to 290 Hz spin frequency. We find that, although it is appropriate to use an arbitrarily complicated model of the similar to 580 Hz oscillations to generate a candidate waveform for the similar to 290 Hz oscillations, models with more than two parameters are not required by the data. For the bursts as a whole, we show that the characteristics of the brightness oscillations vary greatly from burst to burst. We find, however, that in at least one of the bursts, and possibly in three of the four that have strong brightness oscillations throughout the burst, the oscillation frequency decreases well after the peak of the burst. This behavior poses a challenge to the standard burning layer expansion explanation for the frequency changes.