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A. L. Piro (UCSB), L. Bildsten (KITP, UCSB)
Oscillations have now been seen during the type I X-ray bursts of 13 low mass X-ray binaries. These oscillations are characterized by having a frequency of 250-650 Hz, being highly sinusoidal, and showing an upward drift of ~2-5 Hz during the exponentially decaying tail of the outburst. We investigate the hypothesis that these oscillations are nonradial oscillations on the cooling neutron star surface. This environment yields a wide range of interesting modes, including surface waves, g-modes, and crustal interface modes. Such modes must be rotationally modified to match the observed frequencies, implying that the burst oscillations are still a good diagnostic for the neutron star spin. Our work finds additional oscillations may be present during X-ray bursts that have not been previously observed because they exhibit large frequency shifts (~10-100 Hz), so that they have been missed by simple Fourier transforms of the burst lightcurves. Hopefully in the future these will be discovered and investigated by performing systematic \dot{f} searches on the Rossi X-ray Timing Explorer's large database of type I X-ray bursts. The identification of multiple modes on the surface of a neutron star would be a powerful probe of the bursting surface layers and the crystalline crust, and also help to constrain the equation of state for the neutron star core. This work is partially supported by the National Science Foundation under grant PHY99-07949, and by the Joint Institute for Nuclear Astrophysics through NSF grants PHY02-16783 and AST02-05956.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.