Neutron star mass-radius constraints of the quiescent low-mass X-ray binaries X7 and X5 in the globular cluster 47 TUC

Slavko Bogdanov, Craig O. Heinke, Feryal Özel, Tolga Güver

Research output: Contribution to journalArticle

44 Scopus citations

Abstract

We present Chandra/ACIS-S subarray observations of the quiescent neutron star (NS) low-mass X-ray binaries X7 and X5 in the globular cluster 47 Tuc. The large reduction in photon pile-up compared to previous deep exposures enables a substantial improvement in the spectroscopic determination of the NS radius and mass of these NSs. Modeling the thermal emission from the NS surface with a non-magnetized hydrogen atmosphere and accounting for numerous sources of uncertainties, we obtain for the NS in X7 a radius of km for an assumed stellar mass of M = 1.4 M o (68% confidence level). We argue, based on astrophysical grounds, that the presence of a He atmosphere is unlikely for this source. Due to the excision of data affected by eclipses and variable absorption, the quiescent low-mass X-ray binary X5 provides less stringent constraints, leading to a radius of km, assuming a hydrogen atmosphere and a mass of M = 1.4 M o. When combined with all existing spectroscopic radius measurements from other quiescent low-mass X-ray binaries and Type I X-ray bursts, these measurements strongly favor radii in the 9.9-11.2 km range for a ∼1.5 M o NS and point to a dense matter equation of state that is somewhat softer than the nucleonic ones that are consistent with laboratory experiments at low densities.

Original languageEnglish (US)
Article number184
JournalAstrophysical Journal
Volume831
Issue number2
DOIs
StatePublished - Nov 10 2016

Keywords

  • dense matter
  • equation of state
  • globular clusters: individual (47 Tucanae)
  • stars: neutron

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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