Signature of Temporary Burning Front Stalling from a Non-Photospheric Radius Expansion Double-peaked Burst

Abstract

Non-photospheric radius expansion (non-PRE) double-peaked bursts may be explained in terms of spreading ( and temporary stalling) of thermonuclear flames on the neutron star surface, as we argued in a previous study of a burst assuming polar ignition. Here we analyze Rossi X-Ray Timing Explorer ( RXTE) Proportional Counter Array (PCA) data of such a burst (but with a considerably different intensity profile from the previous one) from the low-mass X-ray binary (LMXB) system 4U 1636 - 536 and show that this model can qualitatively explain the observed burst profile and spectral evolution, if we assume an off-polar, but high-latitude, ignition and burning front stalling at a higher latitude compared to that for the previous burst. The off-polar ignition can account for the millisecond period brightness oscillations detected from this burst. This is the first time oscillations have been seen from such a burst. Our model can qualitatively explain the oscillation amplitude measured during the first ( weaker) peak and the absence of oscillations during the second peak. The higher latitude front stalling facilitates the first clear detection of a signature of this stalling, which is the primary result of this work, and may be useful for understanding thermonuclear flame spreading on neutron stars