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Preprint typeset using L ATEX style emulateapj v. 6/22/04 A HYDROGEN ATMOSPHERE SPECTRAL MODEL APPLIED TO THE NEUTRON STAR X7 IN THE GLOBULAR CLUSTER 47 TUCANAE

By George B. Rybicki, Craig O. Heinke, Ramesh Narayan and Jonathan E. Grindlay

Abstract

Current X-ray missions are providing high-quality X-ray spectra from neutron stars (NSs) in quiescent low-mass X-ray binaries (qLMXBs). This has motivated us to calculate fully self-consistent hydrogen-atmosphere models, including opacity due to free-free absorption and Thomson scattering, thermal electron conduction, and self-irradiation by photons from the compact object. We have constructed a grid of neutron star models covering a wide range of surface gravities as well as effective temperatures, which we make available to the scientific community. We present multi-epoch Chandra X-ray observations of the qLMXB X7 in the globular cluster 47 Tuc, which is remarkably nonvariable on timescales from minutes to years. Its high-quality X-ray spectrum is adequately fit by our hydrogen-atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 M ⊙ is assumed, our spectral fits require that its radius km (90 % confidence), larger than expected from currently preferred −0.16 is required. Using models with the appropriate surface gravity for each value of the mass and radius has a significant effect on the allowed parameter space

Year: 2008
OAI identifier: oai:CiteSeerX.psu:10.1.1.317.3389
Provided by: CiteSeerX
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