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C.O. Heinke, J.E. Grindlay, D.A. Lloyd, P.D. Edmonds (Harvard-Smithsonian CfA)
Two quiescent low-mass x-ray binaries (soft x-ray transients), X5 and X7, have been identified in the globular cluster 47 Tuc, and one has been identified in NGC 6397, from Chandra ACIS-I observations. We demonstrate that the unmagnetized neutron star hydrogen-atmosphere models of Lloyd and coworkers, along with the Davis pileup model implemented in XSPEC, give good fits to the spectra of these three neutron stars. Little to no power law component is observed in these spectra, unlike Cen X-4. In addition, we prove that one (X7) of the 47 Tuc neutron stars shows a significantly higher surface gravitational redshift than the other, implying a higher mass. This result probably eliminates neutron star equations of state that display extreme softening.
The author(s) of this abstract have provided an email address for comments about the abstract: cheinke@cfa.harvard.edu