A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae
Heinke, Craig O.
Rybicki, George B.
Grindlay, Jonathan E.
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CitationHeinke, Craig O., George B. Rybicki, Ramesh Narayan, and Jonathan E. Grindlay. 2006. “A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae.” The Astrophysical Journal 644 (2): 1090–1103. https://doi.org/10.1086/503701.
AbstractCurrent 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 new 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 self-consistent 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 multiepoch 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 fitted by our hydrogen atmosphere model without any hard power-law component or narrow spectral features. If a mass of 1.4 M-circle dot is assumed, our spectral fits require that its radius be in the range R-ns =(+1.8)(-1.6) km ( 90% confidence), which is larger than that expected from currently preferred models of NS interiors. If its radius is assumed to be 10 km, then a mass of M-ns = 2.20(-0.16)(+0.03) M-circle dot is required. Using models with the appropriate surface gravity for each value of the mass and radius becomes important for interpretation of the highest quality data.
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