Constraining the mass and radius of neutron stars in globular clusters

Abstract

We analyze observations of eight quiescent low-mass X-ray binaries in globular clusters and combine them to determine the neutron star mass-radius curve and the equation of state of dense matter. We determine the effect that several uncertainties may have on our results, including uncertainties in the distance, the atmosphere composition, the neutron star maximum mass, the neutron star mass distribution, the possible presence of a hotspot on the neutron star surface, and the prior choice for the equation of state of dense matter. We find that the radius of a 1.4 solar mass neutron star is most likely from 10 to 14 km and that tighter constraints are only possible with stronger assumptions about the nature of the neutron stars, the systematics of the observations, or the nature of dense matter. Strong phase transitions are preferred over other models and interpretations of the data with a Bayes factor of 8 or more, and in this case, the radius is likely smaller than 12 km. However, radii larger than 12 km are preferred if the neutron stars have uneven temperature distributions.

Studying strong phase transitions in neutron stars with gravitational waves

“…The detection of the gravitational wave (GW) signal GW170817 [1] by the LIGO [2] and Virgo [3] detectors, followed by electromagnetic (EM) observations [4], opens up the possibility of multi-messenger astronomy as a probe of ultra-dense matter on the quantum chromodynamics (QCD) phase diagram. Estimates of the tidal properties of the binary neutron star (BNS) source of GW170817 [5,6] are inconsistent with too stiff hadronic matter, and lead to NS radii that are consistent with previous X-ray binary observations [7][8][9][10]. With further future BNS detections and improved detector sensitivity [11], GW observations will yield stringent constraints on the NS interior properties [12][13][14][15][16][17][18] that are expected to encounter smaller systematic uncertainties than those from X-ray binaries [19].…”

A Bayesian approach to matching thermonuclear X-ray burst observations with models

“…For mass and radius we also imposed a more informed prior based on the models of Steiner et al (2018), who used observations of eight quiescent low mass X-ray binaries in globular clusters to determine the neutron star mass-radius curve and the equation of state. Steiner et al (2018) used a fixed mass grid and varied radius according to their chosen equation of state and observations of radii. We chose the simplest of their models ('baseline') and used the probability distribution given in Figure 2.4 for the relationship between mass and radius.…”

“…Steiner et al (2018) probability distribution for NS radius as a function of mass based on observations of low mass X-ray binaries in globular clusters. Contour levels shown are 1σ intervals up to 5σ.…”

Gravitational-Wave Constraints on the Neutron-Star-Matter Equation of State

“…These other studies, however, are either based on a small set of individual EOSs (see, e.g., Refs. [13][14][15]), interpret observational data in a way that contains modeling uncertainties [16][17][18][19][20], or apply Bayesian inference to assess the credence of different EOSs based on a theoretical prior [16,[18][19][20]. Some exceptions to this are the works of Hebeler et al, which extrapolates a CET EOS to higher densities [21], and Kurkela et al [6] and Gorda [22], which additionally include a PQCD constraint at high density.…”