Neutron skins and neutron stars in the multi-messenger era (1711.06615v2)

Abstract: The historical first detection of a binary neutron star merger by the LIGO-Virgo collaboration [B. P. Abbott et al. Phys. Rev. Lett. 119, 161101 (2017)] is providing fundamental new insights into the astrophysical site for the $r$-process and on the nature of dense matter. A set of realistic models of the equation of state (EOS) that yield an accurate description of the properties of finite nuclei, support neutron stars of two solar masses, and provide a Lorentz covariant extrapolation to dense matter are used to confront its predictions against tidal polarizabilities extracted from the gravitational-wave data. Given the sensitivity of the gravitational-wave signal to the underlying EOS, limits on the tidal polarizability inferred from the observation translate into constraints on the neutron-star radius. Based on these constraints, models that predict a stiff symmetry energy, and thus large stellar radii, can be ruled out. Indeed, we deduce an upper limit on the radius of a $1.4\,M_{\odot}$ neutron star of $R_{\star}^{1.4}!<!13.76\,{\rm km}$. Given the sensitivity of the neutron-skin thickness of ${}^{208}$Pb to the symmetry energy, albeit at a lower density, we infer a corresponding upper limit of about $R_{\rm skin}^{208}!\lesssim!0.25\,{\rm fm}$. However, if the upcoming PREX-II experiment measures a significantly thicker skin, this may be evidence of a softening of the symmetry energy at high densities---likely indicative of a phase transition in the interior of neutron stars.

• The paper correlates gravitational wave signals from GW170817 with nuclear experiments to constrain the dense matter equation of state.
• It employs relativistic mean-field models to deduce a 1.4 solar mass neutron star radius limit of <13.76 km.
• The findings set an upper limit of ~0.25 fm for the neutron skin thickness of 208Pb, bridging astrophysical and terrestrial data.

Neutron Skins and Neutron Stars in the Multi-Messenger Era

The paper "Neutron skins and neutron stars in the multi-messenger era" by Fattoyev, Piekarewicz, and Horowitz explores the implications of multi-messenger detections involving neutron stars on our understanding of the equation of state (EOS) of dense matter. The authors aim to correlate astronomical observations with nuclear physics experiments, drawing connections that enhance our comprehension of the behavior of nuclear matter under extreme conditions.

Overview

The first historical observation of a binary neutron star (BNS) merger, GW170817, by the LIGO-Virgo collaboration, has opened new avenues for understanding dense matter. The simultaneous detection of gravitational waves (GW) and electromagnetic signals has provided pivotal insights, particularly concerning the EOS constraints of neutron star matter. These observations permit constraints on neutron star properties, notably the tidal polarizability, which in turn relate to pivotal nuclear physics parameters, such as the symmetry energy and neutron skin thickness of atomic nuclei like ${}^{208}$Pb.

Key Findings

The paper discusses how the gravitational wave signal from GW170817 imposes constraints on the tidal polarizability of neutron stars. By modeling the EOS using relativistic mean-field (RMF) approaches, the authors deduce a significant constraint on the neutron star radius. They estimate an upper limit of $R_{\star}^{1.4} < 13.76$ km for a neutron star of 1.4 solar masses. Furthermore, they provide an upper bound for the neutron skin thickness of ${}^{208}$Pb, $R_{\rm skin}^{208} \lesssim 0.25$ fm.

RMF Model Analysis

The analysis employed a set of RMF models, notably including the FSUGold2 family, to explore variations in the symmetry energy's density dependence. The paper finds that models predicting overly stiff EOSs are ruled out based on the tidal polarizability limits derived from GW observations. The models are calibrated against experimental data for finite nuclei and tested for their consistency with neutron star mass-radius observations.

Implications

The implications of this paper are significant for both astrophysics and nuclear physics communities. The BNS merger events, complemented by electromagnetic observations, enhance our ability to probe the EOS of dense matter, impacting our understanding of neutron star structures and properties. Furthermore, the paper indicates how constraints on one type of observable, such as tidal polarizabilities, can translate into indirect constraints on nuclear observables accessible in terrestrial laboratories, such as neutron skin thickness.

Future Prospects

The research suggests potential future developments, particularly with upcoming experiments such as PREX-II, which could measure the neutron skin thickness with greater precision. These results could corroborate or challenge the findings inferred from astrophysical data, potentially suggesting new phases or transitions in neutron star interiors.

Conclusion

This paper exemplifies the synergy between astronomical observations and nuclear experiments, catalyzing advancements in our understanding of high-density nuclear matter. The constraints on neutron star properties derived from the observed GW signal mark a milestone in multi-messenger astronomy. Looking ahead, further observations and improved theoretical models will likely continue refining these constraints, deepening our insights into matter under extreme conditions.