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S. Bhattacharyya (University of Maryland at College Park), T. Strohmayer (NASA/GSFC)
We report on a study of the evolution of burst oscillation properties during the rising phase of X-ray bursts from the low mass X-ray binary system 4U 1636-536 observed with the proportional counter array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE). We present evidence for significant harmonic content of burst oscillation pulses during the early rising phases of bursts. This is the first such detection in burst rise oscillations, and is very important for constraining neutron star structure parameters and the equation of state models of matter at the core of a neutron star. The detection of harmonic content only during the initial portions of the burst rise is consistent with the theoretical expectation that with time the thermonuclear burning region becomes larger, and hence the fundamental and harmonic amplitudes both diminish. We also find, for the first time from this source, strong evidence of oscillation frequency increase during the burst rise. The timing behavior of harmonic content, amplitude, and frequency of burst rise oscillations may provide information about the spreading of thermonuclear flames on neutron star surfaces. Such an information is important for understanding neutron star atmospheres during bursts, which involves complex nuclear physics and geophysical fluid dynamics (with significant stellar spin), under conditions of extreme gravity, magnetic field and radiation pressure. This work was supported in part by NASA Guest Investigator grants.
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Bulletin of the American Astronomical Society, 37 #4
© 2005. The American Astronomical Soceity.