Evidence for harmonic content and frequency evolution of oscillations during the rising phase of X-ray bursts from 4U 1636-536

We report on a study of the evolution of burst oscillation properties during the rising phase of X-ray bursts from 4U 1636-536 observed with the proportional counter array on board the Rossi X- Ray Timing Explorer (RXTE). We present evidence for significant harmonic structure of burst oscillation pulses during the early rising phases of bursts. This is the first such detection in burst rise oscillations and has interesting implications 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 appears 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 frequency increases during burst rise. The timing behavior of harmonic content, amplitude, and frequency of burst rise oscillations may be important in understanding the spreading of thermonuclear flames under the extreme physical conditions on neutron star surfaces.