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Register a new userConstraints on the dense matter equation of state and neutron star properties from NICERs mass-radius estimate of PSR J0740+6620 and multimessenger observations
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In recent years our understanding of the dense matter equation of state (EOS) of neutron stars has significantly improved by analyzing multimessenger data from radio/X-ray pulsars, gravitational wave events, and from nuclear physics constraints. Here we study the additional impact on the EOS from the jointly estimated mass and radius of PSR J0740+6620, presented in Riley et al. (2021) by analyzing a combined dataset from X-ray telescopes NICER and XMM-Newton. We employ two different high-density EOS parameterizations: a piecewise-polytropic (PP) model and a model based on the speed of sound in a neutron star (CS). At nuclear densities these are connected to microscopic calculations of neutron matter based on chiral effective field theory interactions. In addition to the new NICER data for this heavy neutron star, we separately study constraints from the radio timing mass measurement of PSR J0740+6620, the gravitational wave events of binary neutron stars GW190425 and GW170817, and for the latter the associated kilonova AT2017gfo. By combining all these, and the NICER mass-radius estimate of PSR J0030+0451 we find the radius of a 1.4 solar mass neutron star to be constrained to the 95% credible ranges 12.33^{+0.76}_{-0.81} km (PP model) and 12.18^{+0.56}_{-0.79} km (CS model). In addition, we explore different chiral effective field theory calculations and show that the new NICER results provide tight constraints for the pressure of neutron star matter at around twice saturation density, which shows the power of these observations to constrain dense matter interactions at intermediate densities.
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In the past few years, new observations of neutron stars and neutron-star mergers have provided a wealth of data that allow one to constrain the equation of state of nuclear matter at densities above nuclear saturation density. However, most observations were based on neutron stars with masses of about 1.4 solar masses, probing densities up to $sim$ 3-4 times the nuclear saturation density. Even higher densities are probed inside massive neutron stars such as PSR J0740+6620. Very recently, new radio observations provided an update to the mass estimate for PSR J0740+6620 and X-ray observations by the NICER and XMM telescopes constrained its radius. Based on these new measurements, we revisit our previous nuclear-physics multi-messenger astrophysics constraints and derive updated constraints on the equation of state describing the neutron-star interior. By combining astrophysical observations of two radio pulsars, two NICER measurements, the two gravitational-wave detections GW170817 and GW190425, detailed modeling of the kilonova AT2017gfo, as well as the gamma-ray burst GRB170817A, we are able to estimate the radius of a typical 1.4-solar mass neutron star to be $11.94^{+0.76}_{-0.87} rm{km}$ at 90% confidence. Our analysis allows us to revisit the upper bound on the maximum mass of neutron stars and disfavours the presence of a strong first-order phase transition from nuclear matter to exotic forms of matter, such as quark matter, inside neutron stars.
PSR J0740$+$6620 has a gravitational mass of $2.08pm 0.07~M_odot$, which is the highest reliably determined mass of any neutron star. As a result, a measurement of its radius will provide unique insight into the properties of neutron star core matter at high densities. Here we report a radius measurement based on fits of rotating hot spot patterns to Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) X-ray observations. We find that the equatorial circumferential radius of PSR J0740$+$6620 is $13.7^{+2.6}_{-1.5}$ km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030$+$0451, the masses of two other $sim 2~M_odot$ pulsars, and the tidal deformability constraints from two gravitational wave events, to three different frameworks for equation of state modeling, and find consistent results at $sim 1.5-3$ times nuclear saturation density. For a given framework, when all measurements are included the radius of a $1.4~M_odot$ neutron star is known to $pm 4$% (68% credibility) and the radius of a $2.08~M_odot$ neutron star is known to $pm 5$%. The full radius range that spans the $pm 1sigma$ credible intervals of all the radius estimates in the three frameworks is $12.45pm 0.65$ km for a $1.4~M_odot$ neutron star and $12.35pm 0.75$ km for a $2.08~M_odot$ neutron star.
By directly inverting several neutron star observables in the three-dimensional parameter space for the Equation of State of super-dense neutron-rich nuclear matter, we show that the lower radius limit for PSR J0740+6620 of mass $2.08pm 0.07~M_{odot}$ from Neutron Star Interior Composition Explorer (NICER)s very recent observation sets a much tighter lower boundary than previously known for nuclear symmetry energy in the density range of $(1.0sim 3.0)$ times the saturation density $rho_0$ of nuclear matter. The super-soft symmetry energy leading to the formation of proton polarons in this density region of neutron stars is clearly disfavoured by the first radius measurement for the most massive neutron star observed reliably so far.
X-ray pulse profile modeling of PSR J0740+6620, the most massive known pulsar, with data from the NICER and XMM-Newton observatories recently led to a measurement of its radius. We investigate this measurements implications for the neutron star equation of state (EoS), employing a nonparametric EoS model based on Gaussian processes and combining information from other x-ray, radio and gravitational-wave observations of neutron stars. Our analysis mildly disfavors EoSs that support a disconnected hybrid star branch in the mass-radius relation, a proxy for strong phase transitions, with a Bayes factor of $6.9$. For such EoSs, the transition mass from the hadronic to the hybrid branch is constrained to lie outside ($1,2$) $M_{odot}$. We also find that the conformal sound-speed bound is violated inside neutron star cores, which implies that the core matter is strongly interacting. The squared sound speed reaches a maximum of $0.75^{+0.25}_{-0.24}, c^2$ at $3.60^{+2.25}_{-1.89}$ times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740+6620s central density is only $3.57^{+1.3}_{-1.3}$ times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in $beta$-equilibrium.
Recently, the radius of neutron star (NS) PSR J0740+6620 was measured by NICER and an updated measurement of neutron skin thickness of ${}^{208}$Pb ($R_{rm skin}^{208}$) was reported by the PREX-II experiment. These new measurements can help us better understand the unknown equation of state (EoS) of dense matter. In this work, we adopt a hybrid parameterization method, which incorporates the nuclear empirical parameterization and some widely used phenomenological parameterizations, to analyze the results of nuclear experiments and astrophysical observations. With the joint Bayesian analysis of GW170817, PSR J0030+0451, and PSR J0740+6620, the parameters that characterize the ultra dense matter EoS are constrained. We find that the slope parameter $L$ is approximately constrained to $70_{-18}^{+21}$ MeV, which predicts $R_{rm skin}^{208}=0.204^{+0.030}_{-0.026},{rm fm}$ by using the universal relation between $R_{rm skin}^{208}$ and $L$. And the bulk properties of canonical $1.4,M_odot$ NS (e.g., $R_{1.4}$ and $Lambda_{1.4}$) as well as the pressure ($P_{2rho_{rm sat}}$) at two times the nuclear saturation density are well constrained by the data, i.e., $R_{1.4}$, $Lambda_{1.4}$, and $P_{2rho_{rm sat}}$ are approximately constrained to $12.3pm0.7$ km, $330_{-100}^{+140}$, and $4.1_{-1.2}^{+1.5}times10^{34},{rm dyn,cm^{-2}}$, respectively. Besides, we find that the Bayes evidences of the hybrid star and normal NS assumptions are comparable, which indicates that current observation data are compatible with quarkyonic matter existing in the core of massive star. Finally, in the case of normal NS assumption, we obtain a constraint for the maximum mass of nonrotating NS $M_{rm TOV}=2.30^{+0.30}_{-0.18}$ $M_odot$. All of the uncertainties reported above are for 68.3% credible levels.
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