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X-ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State

C. O. Heinke·J. E. Grindlay·D. A. Lloyd·P. D. Edmonds
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Abstract

We report spectral and variability analysis of two quiescent low mass X-ray binaries (X5 and X7, previously detected with the ROSAT HRI) in a Chandra ACIS-I observation of the globular cluster 47 Tuc. X5 demonstrates sharp eclipses with an 8.666+-0.01 hr period, as well as dips showing an increased N_H column. The thermal spectra of X5 and X7 are well-modeled by unmagnetized hydrogen atmospheres of hot neutron stars. No hard power law component is required. A possible edge or absorption feature is identified near 0.64 keV, perhaps an OV edge from a hot wind. Spectral fits imply that X7 is significantly more massive than the canonical 1.4 \Msun neutron star mass, with M>1.8 \Msun for a radius range of 9-14 km, while X5's spectrum is consistent with a neutron star of mass 1.4 \Msun for the same radius range. Alternatively, if much of the X-ray luminosity is due to continuing accretion onto the neutron star surface, the feature may be the 0.87 keV rest-frame absorption complex (O VIII & other metal lines) intrinsic to the neutron star atmosphere, and a mass of 1.4 \Msun for X7 may be allowed.

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C. O. Heinke, J. E. Grindlay, D. A. Lloyd, P. D. Edmonds. 2003-01-14. X-ray Studies of Two Neutron Stars in 47 Tucanae: Toward Constraints on the Equation of State. https://doi.org/10.1086/374039

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