The Frequency Stability of Millisecond Oscillations in Thermonuclear X‐Ray Bursts

We analyze the frequency evolution of millisecond oscillations observedduring type I X-ray bursts with the Rossi X-ray Timing Explorer in order toestablish the stability of the mechanism underlying the oscillations. Oursample contains 68 pulse trains detected in a search of 159 bursts from 8accreting neutron stars. As a first step, we confirm that the oscillationsusually drift upward in frequency by about 1% toward an apparent saturationfrequency. Previously noted anomalies, such as drifts toward lower frequenciesas the oscillations disappear (``spin-down'' episodes) and instances of twosignals present simultaneously at frequencies separated by a few Hz, occur in5% of oscillations. Having verified the generally accepted description of burstoscillations, we proceed to study the coherence of the oscillations duringindividual bursts, and the dispersion in the asymptotic frequencies in burstsobserved over five years. On short time scales, we find that 30% of theoscillation trains do not appear to evolve smoothly in phase. This suggestseither that two signals are present simultaneously with a frequency differencetoo small to resolve (< 1 Hz), that the frequency evolution is discontinuous,or that discrete phase jumps occur. On time scales of years, the maximumfrequencies of the oscillations exhibit fractional dispersions of < 0.004. Inthe case of 4U 1636-536, this dispersion is uncorrelated with the known orbitalphase, which indicates that a mechanism besides orbital Doppler shifts preventsthe oscillations from appearing perfectly stable. We interpret these resultsunder the assumption that the oscillations originate from anisotropies in theemission from the surfaces of these rotating neutron stars. (Abridged)Comment: 13 pages, including 10 figures. To be published in ApJ v580 No. 2, December, 1 2002. Some changes and clarification