Improved mass and radius constraints for quiescent neutron stars in ω Cen and NGC 6397

Abstract

We use Chandra and XMM observations of the globular clusters $\\omega$ Cen and\nNGC 6397 to measure the spectrum of their quiescent neutron stars (NSs), and\nthus to constrain the allowed ranges of mass and radius for each. We also use\nHubble Space Telescope photometry of NGC 6397 to identify a potential optical\ncompanion to the quiescent NS, and find evidence that the companion lacks\nhydrogen. We carefully consider a number of systematic problems, and show that\nthe choices of atmospheric composition, interstellar medium abundances, and\ncluster distances can have important effects on the inferred NS mass and\nradius. We find that for typical NS masses, the radii of both NSs are\nconsistent with the 10-13 km range favored by recent nuclear physics\nexperiments. This removes the evidence suggested by Guillot and collaborators\nfor an unusually small NS radius, which relied upon the small inferred radius\nof the NGC 6397 NS.\n