PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter (1912.05705v1)

Abstract: Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists, because their attributes can be used to determine the properties of the dense matter in their cores. One of the most informative approaches for determining the equation of state of this dense matter is to measure both a star's equatorial circumferential radius $R_e$ and its gravitational mass $M$. Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Explorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are $R_e = 13.02^{+1.24}_{-1.06}$ km and $M = 1.44^{+0.15}_{-0.14}\ M_\odot$ (68%). The independent analysis reported in the companion paper by Riley et al. (2019) explores different emitting spot models, but finds spot shapes and locations and estimates of $R_e$ and $M$ that are consistent with those found in this work. We show that our measurements of $R_e$ and $M$ for PSR J0030$+$0451 improve the astrophysical constraints on the equation of state of cold, catalyzed matter above nuclear saturation density.

• The paper estimates PSR J0030+0451’s mass at 1.44 M☉ and radius at 13.02 km using precise NICER data.
• The research employs a Bayesian inference approach with a three-spot thermal emission model to minimize systematic errors in waveform analysis.
• The findings tighten constraints on the neutron star equation of state by narrowing the pressure-density relationship for dense matter.

Insights from the Mass and Radius Measurement of PSR J0030+0451 Using NICER Data

The paper of neutron stars provides crucial insights into the properties of dense nuclear matter, especially under conditions that cannot be readily replicated in terrestrial laboratories. The paper under discussion presents estimates of the mass and radius of the isolated millisecond pulsar PSR J0030+0451 using data from NASA's Neutron Star Interior Composition Explorer (NICER). This research employs a Bayesian inference approach to analyze the thermal X-ray waveform emitted by the neutron star, a method regarded for its reduced susceptibility to systematic errors compared to other techniques for measuring neutron star radii.

Methodology and Findings

In this work, the authors explored a variety of emission configurations on the stellar surface, ultimately favoring a model featuring three oval, uniformly-temperature emitting spots. This model provided an excellent fit to the NICER observed pulse waveform, yielding mass and radius estimates of $R_e = 13.02^{+1.24}_{-1.06}$ km and $M = 1.44^{+0.15}_{-0.14} M_\odot$ at the 68% confidence level. Furthermore, these results show a consistency with a companion analysis conducted by Riley et al., which applied different spot models and reached consistent findings for radii and mass estimates.

The conclusions drawn about the properties of cold dense matter are noteworthy. The mass and radius of PSR J0030+0451 offer improved constraints on the neutron star equation of state (EoS), in particular on the pressure-density relationship for matter exceeding nuclear saturation density. The paper uses these constraints in conjunction with existing astrophysical data, including high-mass neutron star observations and measurements of tidal deformability from neutron star mergers, to refine our understanding of neutron star matter.

Implications for Neutron Star EoS Modeling

A significant achievement in this paper is the improvement of constraints on the EoS of cold, catalyzed matter via the neutron star's mass-radius relationship. This advances prior efforts by leveraging precise radius measurements to narrow down the feasible range of theoretical models for dense matter. The authors illustrate how their results compare favorably with various hypothesized EoS, emphasizing the narrowing of credible regions for pressures at given densities when coupled with the NICER data.

Considerations and Future Directions

The successful application of a two- and three-spot model emphasizes the potential complexity of heated regions on the neutron star surface, likely influenced by the star's magnetic field configuration. The authors highlight the distinction between the oval spot model and earlier, simpler, circular spot models, showcasing improved fits and parameter estimates.

A crucial aspect of this research is addressing systematic uncertainties in instrumental data. The authors took care to exclude poorly calibrated data below certain energy channels, ensuring that their results are robust against biases originating from instrumental inaccuracies.

The paper proposes several future directions for the paper of neutron star interiors. The continued observation and analysis of other neutron stars using NICER and complementary methods can help refine the EoS further. Additionally, advancing theoretical models to include complex magnetic field configurations and anisotropic surface emissions would likely enhance the interpretation of observed waveforms.

In conclusion, the measurement of PSR J0030+0451's mass and radius utilizing NICER sets a new standard in neutron star research, providing vital constraints on the EoS of dense matter and enhancing our understanding of neutron star properties. The work provides a detailed methodological example for future studies aiming to unravel the mysteries of neutron star interiors and the fundamental nature of matter at extreme densities.