On the Frequency Evolution of X-ray Brightness Oscillations During Thermonuclear X-ray Bursts: Evidence for 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 (LMXB). 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 bursts from 4U 1728-34 and 4U 1702-429 as high as Q ≡ ν0/∆νfwhm ∼ 4000. We use this model of the frequency evolution to search sensitively for significant power at the harmonics and first sub-harmonic of the 330 and 363 Hz signal in bursts from 4U 1702-429 and 4U 1728-23, respectively, but find no strong evidence for 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 sub-harmonic both in bursts from 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 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. Subject headings: X-rays: bursts stars: individual (4U1728-34, 4U1702-429) stars: neutron stars: rotation NASA’s Goddard Space Flight Center, Code 662, Greenbelt, MD 20771; [email protected], [email protected]. National Research Council Resident Associate.