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Session 113 - Neutron Stars & Pulsars.
Oral session, Saturday, January 10
International Ballroom East,

[113.03] Neutron Star Mass-Radius Constraints from the Amplitude of Spin Modulation Pulsations During X-ray Bursts

T. E. Strohmayer (USRA/LHEA/GSFC), J. H. Swank, W. Zhang, N. E. White (LHEA/NASA/GSFC), I. Lapidus (University of Sussex)

There is now strong evidence that the millisecond pulsations observed during the rising phase of thermonuclear X-ray bursts with the Rossi X-ray Timing Explorer (RXTE) are due to spin modulation of a thermonuclear hotspot on the neutron star surface. The maximum intrinsic pulsation amplitude that can be achieved in this scenario is related to the strength of gravitational light bending produced by the neutron star, which is directly related to the ratio of stellar mass to radius, m/r. The intrinsic pulsation amplitude can be degraded by photon scattering and/or the viewing geometry but not increased, thus the observed pulsation amplitude represents an upper limit to the intrinsic pulsation amplitude. This implies that the observed pulsation amplitude places an upper limit on m/r, that is, stars which have mass to radius ratios larger than some value cannot achieve the observed amplitudes. We use RXTE observations of thermonuclear bursts from 4U1636-54 and 4U1728-34 to place constraints on the masses and radii of the neutron stars in these systems. We compare these constraints with those derived from X-ray burst spectroscopy as well as recent RXTE observations of kilohertz QPO.

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