Spin-down of pulsations in the cooling tail of an X-ray burst from 4U 1636-53

With the proportional counter array on board the Rossi X-ray Timing Explore r, we report the discovery of a decrease in the frequency of X-ray brightne ss oscillations in the cooling tail of an X-ray burst from 4U 1636-53. This is the first direct evidence for a spin-down of the pulsations seen during thermonuclear bursts. We find that the spin-down episode is correlated wit h the appearance in this bm:st of an extended tail of emission with a decay timescale much longer than is seen in other bursts from 4U 1636-53 in the same set of observations. We present both time-resolved energy and variabil ity spectra during this burst and compare them with results from a second b urst that shows neither a spin-down episode nor an extended tail. A spectra l evolution study of the "spin-down" burst reveals a secondary signature of weak radius expansion, which is not seen in other bursts and is correlated with the spin-down episode; this may indicate a secondary thermonuclear en ergy release. We interpret the spin-down episode in the context of an angul ar momentum-conserving shell, which is reexpanded and therefore spun down b y an additional thermonuclear energy release that could also explain the ex tended X-ray tail.