On the frequency evolution of X-ray brightness oscillations during thermonuclear X-ray bursts: Evidence of coherent oscillations

We investigate the time dependence of the frequency of X-ray brightness oscillations during thermonuclear X-ray bursts from several neutron star low-mass X-ray binaries. We find that the oscillation frequency in the cooling tails of X-ray bursts from 4U 1702-429 and 4U 1728-34 is well described by an exponential "chirp" model. With this model, we demonstrate that the pulse trains in the cooling tails of many bursts are highly phase coherent. We measure oscillation quality factors for the bursts from 4U 1728-34 and 4U 1702-429 as high as Q = upsilon(0)/Delta upsilon(FWHM) similar to 4000. we use this model of the frequency evolution to search sensitively for significant power at the harmonics and first subharmonic of the 330 and 363 Hz signal in the bursts from 4U 1702-429 and 4U 1728-23, respectively, but we find no strong evidence of significant power at any harmonic or the subharmonic. We argue that the high coherence of the oscillations favors stellar rotation as the source of the oscillations. The lack of a subharmonic in the bursts from both 4U 1728-34 and 4U 1702-429 suggests that in these sources, the burst oscillation frequency is indeed the stellar spin frequency. We briefly discuss the frequency evolution in terms of the rotational motion of an angular momentum-conserving thermonuclear shell. We discuss how the limits on harmonic content can be used to infer properties of the neutron star.