Millisecond oscillations during thermonuclear X-ray bursts
Author(s)Muno, Michael Patrick, 1975-
Massachusetts Institute of Technology. Dept. of Physics.
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I analyze 68 oscillation trains detected in a search of 159 thermonuclear bursts from eight neutron star X-ray binaries observed with the Rossi X-ray Timing Explorer. I use all data that were public as of September 2001. The frequencies of the oscillations are uniformly distributed between 270-620 Hz, and are nearly constant for a given source. They typically have fractional rms amplitudes of 5%. During a burst, the frequencies of the oscillations generally drift upward by -4 Hz to stable limiting values. Neither the amplitudes of the oscillations nor the evolution of their frequencies are simply related to the time scales and energetics of the bursts. If the frequency drift is accounted for with smooth functions, the oscillations are coherent in 70% of the bursts, and their asymptotic frequencies are stable to a few parts in 1000. This suggests that the asymptotic frequencies are determined by the spin of the neutron star. However, residual dispersion in the frequencies from 4U 1636-536 is uncorrelated with the known orbit of the system, so some mechanism other than the orbital motion of the neutron star must cause them to vary. Models for the oscillations suggest that they originate from patterns in the stellar surface brightness that drift in the direction opposite the rotation of the neutron star, yielding oscillation frequencies lower than that of the spin. The patterns produce oscillations that are very sinusoidal - harmonic components have amplitudes that are less than 5-10% of those of the main signals. This suggests that the patterns are very symmetric. The types of bursts that exhibit oscillations are correlated with the pulsation frequency.(cont.) In particular, oscillations between 500-650 Hz occur almost exclusively in bursts that exhibit photospheric radius expansion, while oscillations with frequencies of 250-400 Hz are slightly more likely to occur in bursts without radius expansion. This appears to be because (i) oscillations from all of the sources are observed only when the persistent accretion rate is relatively high (- 0.1lMEdd), while (ii) in - 300 Hz sources radius expansion occurs at lower M, yet in - 600 Hz sources it occurs at higher M.
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 2002.Also issued in leaves.Includes bibliographical references (p. 113-121).
DepartmentMassachusetts Institute of Technology. Dept. of Physics.
Massachusetts Institute of Technology