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M.C. Miller (University of Chicago)
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 580 Hz brightness oscillations during five bursts from 4U 1636-536. 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 reaches a maximum several seconds into the burst and then decreases. This behavior has not been reported previously for burst brightness oscillations, and it poses a challenge to the standard layer expansion explanation for the frequency changes. This work was supported in part by NASA grant NAG 5-2868, NASA AXAF contract SV 464006, and NASA ATP grant NRA-98-03-ATP-028.
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