A non-pre double-peaked burst from 4U 1636-536: Evidence of burning front propagation

We analyze Rossi X- Ray Timing Explorer ( RXTE) Proportional Counter Array ( PCA) data of a double- peaked burst from the low- mass X- ray binary ( LMXB) 4U 1636 - 536 that shows no evidence of photospheric radius expansion ( PRE). We find that the X- ray - emitting area on the star increases with time as the burst progresses, even though the photosphere does not expand. We argue that this is a strong indication of thermonuclear flame spreading on the stellar surface during such bursts. We propose a model for such double- peaked bursts, based on thermonuclear flame spreading, that can qualitatively explain their essential features as well as the rarity of these bursts.