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

Abstract

X-ray pulse profile modeling of PSR J0740 þ 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 measurement's 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 gravitationalwave 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 ð1; 2Þ M ⊙ . 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 þ0.25 −0.24 c 2 at 3.60 þ2.25 −1.89 times nuclear saturation density at 90% credibility. Since all but the gravitational-wave observations prefer a relatively stiff EoS, PSR J0740 þ 6620's central density is only 3.57 þ1.3 −1.3 times nuclear saturation, limiting the density range probed by observations of cold, nonrotating neutron stars in β-equilibrium.

Large and massive neutron stars: Implications for the sound speed in dense QCD

“…error bars) [26]. The microscopic calculations used in this work tend to predict somewhat stiffer EOSs compared to other recent microscopic calculations, but still lie on the softer side of the theory-agnostic constraints by Legred et al [27] (red error bars). For instance, in PNM at N 2 LO, the EOS considered here predicts P (2 n sat ) ≈ (20.6 ± 6.6) MeV fm −3 at the 1σ confidence level, while state-of-the-art QMC calculations based on a different set of local χEFT NN and 3N interactions obtained P (2 n sat ) ≈ (15.1 ± 4.7) MeV fm −3 (see Table 2 in Ref.…”

“…4), the largest possible R 2.0 limited by causality is then ≈ 13.53 km (13.87 km) for n c = 2.0 n sat (1.8 n sat ) (where the green dashed curves intersect with the upper y-axis), and a measurement of R 2.0 13 km indicates min{c 2 s,max } 0.42 (0.35), which violates the conformal bound c 2 s 1/3 [10,16,17,49]. Note that at this stage we have not yet taken into account tidal deformability constraints from GW170817 [44] (represented by the gray regions that are excluded) which dominate the upper limit on the stiffness or pressure of EOS at intermediate densities (2 − 3) n sat [10,27].…”

“…[20,21] for the interaction with the momentum cutoff 500 MeV ["GP-B (500 MeV)"]. The error bars show the microscopic constraints from Hebeler et al [25] (based on lower-order MBPT calculations) and Lonardoni et al [26] (based on QMC calculations) as well as the theory-agnostic constraints from neutron star observation by Legred et al [27]. The green, blue, and purple lines depict the phenomenological EOSs NRAPR, SkAPR, and APR in beta equilibrium [28,29], respectively.…”

Implicit correlations within phenomenological parametric models of the neutron star equation of state

“…We expect GW measurements of high-mass NSs (≥ 1.7M ) to be less informative than lower mass NSs, as the absolute impact of tidal parameters on the signal is weaker for more compact stars. In general, high mass NSs will most likely be indistinguishable from black holes until the advent of next-generation detectors [27,54]. As such, high-mass systems offer little information for either M max or R 1.4 , and thus the GW data primarily probe only the lowmass/low-density part of the EoS.…”

Investigating Signatures of Phase Transitions in Neutron-Star Cores

“…Neutron stars (NSs), i.e., radio [1][2][3][4], X-ray [5][6][7][8], gravitational-wave (GW) observations [9,10], and their electromagnetic (EM) counterparts [11,12], have provided valuable new insights into the equation of state (EoS) of dense matter [7,[13][14][15][16][17][18][19][20]. Nevertheless, the composition of matter at several times nuclear saturation density (n sat ≈ 0.16 fm −3 ) remains largely unknown [21] but might be elucidated by future multimessenger data.…”