Neutron Star Mass-Radius Constraints of the Quiescent Low-mass X-ray Binaries X7 and X5 in the Globular Cluster 47 Tuc

TL;DR

This study uses Chandra X-ray observations to precisely measure neutron star radii in two low-mass X-ray binaries, providing key constraints on the dense matter equation of state.

Contribution

It offers improved radius measurements of neutron stars in X7 and X5, reducing uncertainties and refining constraints on the dense matter equation of state.

Findings

Neutron star in X7 has radius 11.1 km with 68% confidence.

Neutron star in X5 has radius 9.6 km with uncertainties.

Results favor a softer dense matter equation of state.

Abstract

We present Chandra ACIS-S sub-array observations of the quiescent neutron star 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 neutron star radius and mass of these neutron stars. Modeling the thermal emission from the neutron star surface with a non-magnetized hydrogen atmosphere and accounting for numerous sources of uncertainties, we obtain for the neutron star in X7 a radius of $R=11.1^{+0.8}_{-0.7}$ km for an assumed stellar mass of $M=1.4$ M$_{\odot}$ (68% C.L.). We argue, based on astrophysical grounds, that the presence of a He atmosphere is unlikely for this source. Due to eclipses and variable absorption, the quiescent low-mass X-ray binary X5 provides less stringent constraints, leading to a radius of $R=9.6^{+0.9}_{-1.1}$ km, assuming a hydrogen atmosphere and a mass of $M=1.4$ M$_{\odot}$. When combined with all other existing spectroscopic radius measurements, these measurements strongly favor radii in the 9.9-11.2 km range for a ~1.5 M$_{\odot}$ neutron star 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.