A Non-pre Double-peaked Burst from 4u 1636–536: Evidence for Burning Front Propagation

We analyse 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 for 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. Subject headings: accretion, accretion disks — relativity — stars: neutron — X-rays: binaries — X-rays: bursts — X-rays: individual (4U 1636–536)