Impact of the PSR J0740+6620 radius constraint on the properties of high-density matter

Abstract

X-ray pulse profile modeling of PSR J0740+6620, the most massive known pulsar, by the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurement's implications for the neutron star equation of state, employing a nonparametric EoS model based on Gaussian processes and combining information from other X-ray, radio and gravitationalwave observations of neutron stars. Our analysis mildly disfavors equations of state that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of 6.4. For EoSs with multiple stable branches, the transition mass from the hadronic to the hybrid branch is constrained to lie outside (1, 2) M . We also find that the conformal sound-speed bound is violated inside neutron star cores, implying that core matter is strongly interacting. The squared sound speed reaches a maximum of 0.79 +0.21 −0.26 c 2 at 3.51 +2.30 −1.76 times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740+6620's central density is only 3.0 +1.6 −1.6 times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in β-equilibrium.

Investigating the mass-ratio dependence of the prompt-collapse threshold with numerical-relativity simulations

“…Their predicted radii for a 1.4 M NS are 11.46 km, 12.38 km, and 13.54 km. Hence, the chosen EOSs are broadly compatible with recent maximum mass and radius constraints, e.g., [44][45][46][47][48][49][50][51][52][53][54] 4 , but also cover a reasonably large range of compactnesses.…”

Relativistic hybrid stars in light of the NICER PSR J0740+6620 radius measurement

“…The lower bound on the radius of PSR J0740 constrains a highly relevant region of the mass-radius (M-R) diagram of compact stars (CS), which in combination with the determination of the tidal deformability (TD) of a star of mass ∼ 1.4M in the GW170817 event by the LIGO-Virgo Collaboration (Abbott et al 2019) put significant constraints on the EoS of CS. These require that the stellar EoS must be moderately soft at intermediate densities (to allow for relatively small TDs) and must be stiff enough at high densities (to allow for two-solar mass CSs) [see (Tan et al 2021;Biswas 2021;Legred et al 2021;Raaijmakers et al 2021;Huth et al 2021;Zhang & Li 2021;Tang et al 2021)]. Recently, the Lead Radius Experiment Collaboration (PREX-II) reported the most precise measurement yet of the neutron skin thickness of a heavy nucleus (Adhikari et al 2021), a quantity strongly correlated with the slope of nuclear symmetric energy L sym at saturation density (ρ sat ), which constrains the nuclear EoS at densities ≥ ρ sat .…”

“…Confronting the very massive star's radius estimation with the possible phase transition in dense matter is thus very timely. This issue has been discussed by Legred et al (2021); Tan et al (2021).…”

Continuum, cyclotron line, and absorption variability in the high-mass X-ray binary Vela X-1

“…We use ω = 1 (assuming that the spectrum inside the emission region is dominated by Bremsstrahlung). Recent publications (Legred et al 2021) tend to assume a typical neutron star radius R NS around 12 km. However, for this work, we used R NS = 10 km under the same assumptions as in Becker et al (2012) for typical neutron star parameters 5 .…”