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

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 NH 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 M⊙ neutron star mass, with M> 1.8M⊙ for a radius range of 9-14 km, while X5’s spectrum is consistent with a neutron star of mass 1.4M⊙ 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 M⊙ for X7 may be allowed. Subject headings: accretion disks — binaries: close, eclipsing — binaries: X-ray — globular clusters: individual (NGC 104) — stars: neutron