A Hydrogen Atmosphere Spectral Model Applied to the Neutron Star X7 in the Globular Cluster 47 Tucanae

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 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.