Neutron Star Mass–radius Constraints of the Quiescent Low-Mass X-Ray Binaries X7 and X5 in the Globular Cluster 47 Tuc

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 = -+ R 11.1 0.7 0.8 km for an assumed stellar mass of M=1.4 M e (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 e . 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 e 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.

Chandra studies of the globular cluster 47 Tucanae: a deeper X-ray source catalogue, five new X-ray counterparts to millisecond radio pulsars and new constraints to r-mode instability window

“…9. Sources with higher counts in the 2-6 keV range, notably W42 (X9), seem to show an increased luminosity in this work, whereas sources with lesser counts in this hard band, notably W46 (X7), seem to show a decreased luminosity (though spectral fitting shows that W46/X7 has demonstrated that it is not actually variable; Bogdanov et al 2016). A likely explanation is the reduced sensitivity of the ACIS-S detector in the lower energy bands, which combined with a single assumed spectral model can produce such apparent variations.…”

“…While the five 2000 observations (described in Grindlay et al 2001a) were carried out with the ACIS-I CCD array at telescope focus, the eight 2002 observations (described in H05) and the six 2014-15 observations (described in Bogdanov et al 2016) were acquired with the ACIS-S CCD array. All the 2014-15 observations, as well as short observations in 2000 and 2002, were taken using a subarray, which reduced the frame time, and thus reduced "pile-up"-the (incorrect) recording of two photons which landed on nearby pixels during one frame as a single event.…”

“…(Bogdanov et al 2016) with those made in 2000 and 2002, to obtain a deeper image of 47 Tuc with improved angular resolution. In this paper, we describe the X-ray analysis we used to create a larger source catalogue with accurate source positions, and focus on the X-ray properties of the MSPs 47 Tuc F, S, Z, aa and ab, whose X-ray counterparts we have identified in this work.…”

“…It is considered to possess a significant population of binaries whose properties have been altered by close encounters with other stars or binaries, by virtue of being one of the clusters with the highest predicted close encounter frequencies (Verbunt & Hut 1987;Pooley et al 2003). We use a cluster absorbing column of 3.5×10 20 cm− 2 (Harris 1996(Harris , 2010, a distance of 4.53 kpc (Bogdanov et al 2016;Hansen et al 2013), and metal abundances compiled in Heinke et al (2006). Measuring the various binary populations of clusters like 47 Tuc is crucial for understanding and modelling the dynamical encounters between binaries that produce X-ray sources in globular clusters (e.g.…”

From Neutron Star Observables to the Equation of State. I. An Optimal Parametrization

“…This can be achieved by comparing measured masses (Demorest et al 2010;Antoniadis et al 2013) and radii (Guillot et al 2013;Guillot & Rutledge 2014;Heinke et al 2014;Nättilä et al 2015;Bogdanov et al 2016;; see Özel & Freire 2016 for a recent review) to the predictions of particular EOS. However, this approach is limited in scope, as it serves to constrain the parameters of already formulated EOS frameworks and may not necessarily span the full range of possibilities.…”