2016ApJ...831..184B -
Astrophys. J., 831, 184-184 (2016/November-2)
Neutron star mass-radius constraints of the quiescent low-mass X-ray binaries X7 and X5 in the globular cluster 47 Tuc.
BOGDANOV S., HEINKE C.O., OZEL F. and GUVER T.
Abstract (from CDS):
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 R=11.1–0.7+0.8 km for an assumed stellar mass of M = 1.4 M☉ (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 R=9.6–1.1+0.9 km, assuming a hydrogen atmosphere and a mass of M = 1.4 M☉. 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☉ 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.
Abstract Copyright:
© 2016. The American Astronomical Society. All rights reserved.
Journal keyword(s):
dense matter - equation of state - globular clusters: individual: 47 Tucanae - stars: neutron - stars: neutron
Simbad objects:
21
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