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 O V 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 > 1.8 M☉ for a radius range of 9-14 km, while X5's spectrum is consistent with a neutron star of mass 1.4 M☉ 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 and other metal lines) intrinsic to the neutron star atmosphere, and a mass of 1.4 M☉ for X7 may be allowed.