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D.K. Galloway (MIT), A. Cumming (UC Santa Cruz), D. Chakrabarty (MIT)
Five millisecond X-ray pulsars are presently known, two of which have been found to exhibit thermonuclear (type-I) X-ray bursts. We present a comparison of the properties of the bursts with theoretical ignition models, with a view towards constraining the composition of the fuel. The bursts from XTE J1814-338 are irregular and appear to be too frequent for the inferred accretion rate, unless the source is substantially more distant than estimated or that accretion may be only over part of the neutron star surface. The recurrence time for the 4 bursts from SAX J1808.4-3658 on the other hand decrease proportionately with the accretion rate, and suggest slightly H-deficient fuel. Sub-solar H abundance may be intrinsic to the accreted fuel or the result of chemical fractionation in the settling atmosphere.
The other three pulsars are in "ultracompact" binary systems with orbital periods of ~40 min and evolved (H-poor) donors, and have not shown bursts. We have also used the predictions of burst models to attempt to determine upper limits on the H abundance in the fuel layer in those sources. In XTE J1751-305 we derive a marginally significant upper limit on the H-fraction of 0.3.
This work was supported in part by the NASA Long Term Space Astrophysics program under grant NAG 5-9184. AC is supported by NASA Hubble Fellowship grant HF-01138.
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Bulletin of the American Astronomical Society, 36 #3
© 2004. The American Astronomical Soceity.