Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory (1908.10352v3)

Abstract: The properties of neutron stars are determined by the nature of the matter that they contain. These properties can be constrained by measurements of the star's size. We obtain stringent constraints on neutron-star radii by combining multimessenger observations of the binary neutron-star merger GW170817 with nuclear theory that best accounts for density-dependent uncertainties in the equation of state. We construct equations of state constrained by chiral effective field theory and marginalize over these using the gravitational-wave observations. Combining this with the electromagnetic observations of the merger remnant that imply the presence of a short-lived hyper-massive neutron star, we find that the radius of a $1.4\,\rm{M}\odot$ neutron star is $R{1.4\,\mathrm{M}\odot} = 11.0^{+0.9}{-0.6}~{\rm km}$ (90% credible interval). Using this constraint, we show that neutron stars are unlikely to be disrupted in neutron-star black-hole mergers; subsequently, such events will not produce observable electromagnetic emission.

• The paper combines multimessenger observations with chiral effective field theory to tightly constrain the radii of 1.4 solar mass neutron stars.
• It determines a neutron star radius of 11.0 km with uncertainties of +0.9 km and -0.6 km, significantly reducing previous error margins.
• The findings refine predictions for tidal interactions in neutron-star–black-hole mergers, influencing expectations for electromagnetic emissions.

Insights on Neutron Star Radius Constraints from Multimessenger Data and Nuclear Theory

The research paper "Stringent constraints on neutron-star radii from multimessenger observations and nuclear theory" provides a detailed analysis of neutron star properties by leveraging data from the binary neutron-star merger event, GW170817. This paper uniquely combines multimessenger observational data with advanced nuclear theory to derive tight constraints on neutron-star dimensions, particularly focusing on the radius of a neutron star with 1.4 solar masses, $R_{1.4\,M_\odot}$.

Combining Observational Data and Theoretical Models

The crux of this research lies in the combination of observational data from the merger event GW170817, including gravitational waves and electromagnetic signals, with nuclear equations of state derived from chiral effective field theory. The paper emphasizes that the equations of state (EOS) are constrained not only by experimental nuclear data but also through effective field theory methods that naturally incorporate theoretical uncertainties that increase with density. This approach allows the derivation of equations of state that remain valid up to certain nuclear saturation densities, providing a robust framework to interpret neutron star observations.

Key Findings and Implications

1. Neutron Star Radius Measurement: By incorporating constraints from GW170817’s gravitational-wave data and electromagnetic counterparts, the authors determine the radius of a 1.4 solar mass neutron star to be $R_{1.4\,M_\odot} = 11.0^{+0.9}_{-0.6}$ km. This result sets a stringent limit, outperforming previous estimates with a reduction in uncertainty by a factor of approximately two.
2. Constraints from Nuclear Physics: The authors emphasize that previously, parametrizations of the neutron star EOS often ignored or inadequately represented the density-dependent uncertainties inherent in current nuclear theoretical models. This paper's application of chiral effective field theory to quantify and manage these uncertainties marks a significant methodological advancement.
3. Implications for Neutron-Star–Black-Hole Merger Observations: One of the practical implications of the constrained neutron star radii is related to the tidal interactions in neutron-star–black-hole mergers. The research suggests that neutron stars with the derived radius are unlikely to be disrupted in such mergers unless the black hole's mass is exceptionally low. Consequently, these events may not produce observable electromagnetic emissions, refining the predictions and strategies for future observational campaigns.
4. Pressure Constraints and Theoretical Validation: The paper quantifies the pressure conditions inside neutron stars, with specific analyses of pressure at multiple times the nuclear saturation density. The results conform to previous predictions that the speed of sound must exceed $c/\sqrt{3}$ in dense stellar interiors, supporting specific theoretical models pertaining to the behavior of ultra-dense matter.

Future Research Directions

The paper also touches on several areas that warrant future research. Future developments could include refining waveform models used for gravitational wave observations, enhancing the precision of radius measurements, and improving our understanding of dense matter through higher-fidelity simulations. Additionally, improved nuclear interaction models and calculations could provide further insights into matter behavior at densities beyond the breakdown of current theoretical frameworks.

In summary, this research significantly advances the field by providing more reliable estimates of neutron star radii through an innovative integration of observational data and robust nuclear theory. The implications extend across astrophysics and nuclear physics, providing a tighter nexus between theory and observation and guiding future explorations of compact stellar phenomena.