Progress in Constraining Nuclear Symmetry Energy Using Neutron Star Observables Since GW170817

Abstract

The density dependence of nuclear symmetry energy is among the most uncertain parts of the Equation of State (EOS) of dense neutron-rich nuclear matter. It is currently poorly known especially at suprasaturation densities partially because of our poor knowledge about isovector nuclear interactions at short distances. Because of its broad impacts on many interesting issues, to pin down the density dependence of nuclear symmetry energy has been a longstanding and shared goal of both astrophysics and nuclear physics. New observational data of neutron stars including their masses, radii, and tidal deformations since GW170817 have helped improve our knowledge about nuclear symmetry energy especially at high densities. Based on various model analyses of these new data by many people in the nuclear astrophysics community, while our brief review might be incomplete and biased unintentionally, we have learned particularly: (1) The slope parameter L of nuclear symmetry energy at saturation density ρ 0 of nuclear matter from 24 new analyses of neutron star observables is about L ≈ 57.7 ± 19 MeV at 68% confidence level consistent with its fiducial value from surveys of over 50 earlier analyses of both terrestrial and astrophysical data within error bars, (2) The curvature K sym of nuclear symmetry energy at ρ 0 from 16 new analyses of neutron star observables is about K sym ≈ −107 ± 88 MeV at 68% confidence level in very good agreement with the systematics of earlier analyses, (3) The magnitude of nuclear symmetry energy at 2ρ 0 , i.e. E sym (2ρ 0 ) ≈ 51 ± 13 MeV at 68% confidence level, has been extracted from 9 new analyses of neutron star observables consistent with results from earlier analyses of heavy-ion reactions and the latest predictions of the state-of-the-art nuclear many-body theories, (4) while the available data from canonical neutron stars do not provide tight constraints on nuclear symmetry energy at densities above about 2ρ 0 , the lower radius boundary R 2.01 = 12.2 km from NICER's very recent observation of PSR J0740+6620 of mass 2.08 ± 0.07 M and radius R = 12.2 − 16.3 km at 68% confidence level sets a tight lower limit for nuclear symmetry energy at densities above 2ρ 0 , (5) Bayesian inferences of nuclear symmetry energy using models encapsulating a first-order hadron-quark phase transition from observables of canonical neutron stars indicate that the phase transition shifts appreciably both the L and K sym to higher values but with larger uncertainties compared to analyses assuming no such phase transition, (6) The high-density behavior of nuclear symmetry energy affects significantly the minimum frequency necessary to rotationally support GW190814's secondary component of mass (2.50-2.67) M as the fastest and most massive pulsar discovered so far. Overall, thanks to the hard work of many people in the astrophysics and nuclear physics community, new data of neutron star observations since the discovery of GW170817 have enriched significantly our knowledge about the symmetry energy of dense neutron-ric...

Impact of NICER's Radius Measurement of PSR J0740+6620 on Nuclear Symmetry Energy at Suprasaturation Densities

“…These constraints are quantitatively more accurate than our previous results obtained using earlier NS observations (Zhang & Li 2019a). Moreover, a recent survey of 9 independent analyses of NS observations since GW170817 and 2 earlier analyses of heavy-ion reaction experiments gave a fiducial value of E sym (2ρ 0 ) ≈ 51 ± 13 MeV at 68% confidence level (Li et al 2021). The E sym (2ρ 0 ) = 50.55 ± 5.99 MeV from the present work is consistent with this fiducial value but has an about 50% smaller error bar, indicating a significant refinement due to NICER's radius measurement of PSR J0740+6620.…”

“…where E 0 (ρ 0 ) = −15.9 ± 0.4 MeV is the binding energy and K 0 ≈ 240 ± 20 MeV (Shlomo et al 2006;Piekarewicz 2010;Garg & Colò 2018) is the incompressibility at the saturation density ρ 0 of SNM, while E sym (ρ 0 ) = 31.7 ± 3.2 MeV is the magnitude and L ≈ 58.7 ± 28.1 MeV is the slope of symmetry energy at ρ 0 (Li & Han 2013;Oertel et al 2017) based on earlier surveys of over 50 analyses of both terrestrial experiments and astrophysical observations, respectively. A very recent survey of 24 new analyses of NS observations since GW179817 indicates that L ≈ 57.7 ± 19 MeV and K sym ≈ −107 ± 88 MeV at 68% confidence level (Li et al 2021). In this study, we use L = 58.7 as its most probable value and vary it within ±20 MeV.…”

“…In the above discussions, the slope L of symmetry energy is set at its fiducial value of L = 58.7 MeV, while it is known that it still has an uncertainty of about 20 MeV based on the latest survey of available analyses in the literature (Li et al 2021). The L parameter characterizes mainly the symmetry energy around ρ 0 .…”

“…Combined with NICER's earlier simultaneous mass and radius measurement of PSR J0030+0451 (Miller et al 2019;Riley et al 2019), the first radius measurement of the most massive NS has the strong potential to reveal interesting new physics about the Equation of State (EOS) of super-dense neutron-rich nuclear matter. Besides earlier predictions about what uniquely new physics can be learned from the radii of massive NSs compared to canonical ones, see, e.g., Xie & Li (2020); Han & Prakash (2020); Drischler et al (2021); Somasundaram & Margueron (2021), several new analyses aiming at extracting new information about the EOS of super-dense matter from NS observations including the latest NICER observations have already been carried out (Biswas 2021;Li et al 2021;Raaijmakers et al 2021;Pang et al 2021). In this Letter, we show that the 68% lower mass-radius boundary of R 2.01 ≥ 12.2 km (Miller et al 2021) provides a much tighter lower boundary than previously known for nuclear symmetry energy in the density range of (1.0 ∼ 3.0)ρ 0 .…”

Hyperonization in compact stars

“…;;;Cai and Li (2021);Li and Magno (2020);Li et al (2021)]. Detailed investigations of the EoS were carried out where the higher-order coefficients of the expansion, specifically, Q sat and L sym were varied since their values are weakly constrained by the conventional fitting protocol of CDF[Margueron et al (2018a,b);Margueron and Gulminelli (2019);Zhang et al (2018);Li and Sedrakian (…”