Masses, Radii, and Equation of State of Neutron Stars (1603.02698v1)

Abstract: We summarize our current knowledge of neutron star masses and radii. Recent instrumentation and computational advances have resulted in a rapid increase in the discovery rate and precise timing of radio pulsars in binaries in the last few years, leading to a large number of mass measurements. These discoveries show that the neutron star mass distribution is much wider than previously thought, with 3 known pulsars now firmly in the 1.9-2.0 Msun mass range. For radii, large, high quality datasets from X-ray satellites as well as significant progress in theoretical modeling led to considerable progress in the measurements, placing them in the 9.9-11.2 km range and shrinking their uncertainties due to a better understanding of the sources of systematic errors. The combination of the massive neutron star discoveries, the tighter radius measurements, and improved laboratory constraints of the properties of dense matter has already made a substantial impact on our understanding of the composition and bulk properties of cold nuclear matter at densities higher than that of the atomic nucleus, a major unsolved problem in modern physics.

• The paper presents a thorough review of neutron star mass and radius measurements, highlighting nearly 2.0 M☉ findings that constrain dense matter's EoS.
• It details advances in observational techniques, from precise pulsar timing to refined X-ray emission modeling, that reduce measurement uncertainties.
• The study discusses future prospects with instruments like SKA and NICER to further probe extreme physics and validate theoretical EoS models.

An Expert Review of Neutron Star Masses, Radii, and Equations of State

This paper provides a comprehensive review of the observational and theoretical understanding of neutron stars, focusing on the precise measurements of their masses, radii, and implications for the equation of state (EoS) of dense matter. The authors, Feryal Özel and Paulo Freire, address recent advances propelled by both technological innovations in observational astronomy and theoretical developments in nuclear physics and astrophysics.

Overview of Neutron Star Mass and Radius Measurements

The paper emphasizes the broad distribution of neutron star masses, which has been more extensive than previously thought. A sizable number of neutron stars with masses nearing 2.0 $M_{\odot}$ have been identified. These discoveries have crucial implications for constraining the EoS, especially at densities several times higher than nuclear saturation density. Measurements indicate that the radii of several neutron stars lie between 9.9 and 11.2 km, with shrinking uncertainties due to refined observational techniques and a better understanding of systematic errors.

Implications for the Equation of State

The unique conditions in the cores of neutron stars make them an exceptional natural laboratory for dense matter physics. Neutron stars probe regimes unattainable in terrestrial laboratories, reaching densities up to 8 times the nuclear density. The combination of massive neutron stars and robust radius measurements provides significant constraints on theoretical models predicting neutron star compositions, such as the transition from nuclear matter to quark matter, presence of hyperons, or other exotic states. Different theoretical models, including those incorporating quark matter or hyperonic states, predict varied mass-radius relations, with some leading to significantly softer EoS that may be inconsistent with these observations, particularly when massive neutron stars above 2.0 $M_{\odot}$ are considered.

Observational Techniques and Advances

A detailed discussion is included on the methods employed to measure neutron star masses and radii. Pulsar timing, using radio observations, has yielded precise mass measurements, particularly for pulsars in binary systems. The authors provide extensive tables cataloging these masses, highlighting the diverse methods and systems in which precise timing has been achieved.

For radii measurements, the paper reviews techniques involving X-ray observations of thermal emission from neutron star surfaces, either in quiescence or during X-ray bursts. The intricacies involved in modeling neutron star atmospheres and accounting for gravitational lensing effects are thoroughly discussed, underlining the importance of these considerations in extracting reliable radius measurements.

Future Prospects

The review anticipates further progress in neutron star mass and radius measurements, emphasizing the potential for a broader application of emerging observational instruments like the Square Kilometer Array (SKA) and X-ray missions such as NICER. Improvements in measuring distances to neutron star binaries and advances in gravitational wave detections via facilities like Advanced LIGO are expected to refine our understanding of neutron star structure further. In particular, the potential measurement of the moment of inertia in systems like the double pulsar offers promising prospects for testing general relativity in strong gravitational fields and constraining the EoS.

Conclusion

This work underscores the pivotal role of neutron star observations in shaping our understanding of dense matter. The authors present the current state of research with rigor, acknowledging the multifaceted approach required to address the complexities of neutron star interiors. The synthesis of observational data with theoretical models holds profound implications for fields as varied as nuclear physics, astrophysics, and general relativity, paving the way for future discoveries. The paper positions neutron stars as key to unraveling the secrets of the EoS—a major unsolved problem in contemporary physics—with anticipated advancements likely to propel further insights into this enigmatic domain.