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A. L. Watts, T. E. Strohmayer (NASA GSFC)
High frequency oscillations are seen in both the rise and tail of Type I X-ray bursts. Recent observations of accreting millisecond pulsars confirm that the frequency is close to the spin frequency of the neutron star, although the frequency can shift by ~1% during some bursts. The modulation amplitude is highest at burst onset, reducing during the burst rise, but can remain as high as 15% in the burst tail. Any viable mechanism must explain all of these observations. In the burst rise, the asymmetry can be understood as a localized 'hot spot' induced by thermonuclear ignition and subsequent spreading. The source of the asymmetry in the burst tail, however, remains unclear. One possibility is that modes of oscillation in the surface layers of the neutron star are excited by the burning front, the resulting mode pattern giving a brightness asymmetry. We report results from an ongoing investigation into the viability of this mechanism. Of particular interest is the mode amplitude that would be necessary to account for the observed flux variations, and the effect of atmospheric shearing on mode frequency.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.