A NICER View of the Nearest and Brightest Millisecond Pulsar: PSR J0437$\unicode{x2013}$4715 (2407.06789v1)

Abstract: We report Bayesian inference of the mass, radius and hot X-ray emitting region properties - using data from the Neutron Star Interior Composition ExploreR (NICER) - for the brightest rotation-powered millisecond X-ray pulsar PSR J0437$\unicode{x2013}$4715. Our modeling is conditional on informative tight priors on mass, distance and binary inclination obtained from radio pulsar timing using the Parkes Pulsar Timing Array (PPTA) (Reardon et al. 2024), and we use NICER background models to constrain the non-source background, cross-checking with data from XMM-Newton. We assume two distinct hot emitting regions, and various parameterized hot region geometries that are defined in terms of overlapping circles; while simplified, these capture many of the possibilities suggested by detailed modeling of return current heating. For the preferred model identified by our analysis we infer a mass of $M = 1.418 \pm 0.037$ M$\odot$ (largely informed by the PPTA mass prior) and an equatorial radius of $R = 11.36^{+0.95}{-0.63}$ km, each reported as the posterior credible interval bounded by the 16% and 84% quantiles. This radius favors softer dense matter equations of state and is highly consistent with constraints derived from gravitational wave measurements of neutron star binary mergers. The hot regions are inferred to be non-antipodal, and hence inconsistent with a pure centered dipole magnetic field.

A NICER View of the Nearest and Brightest Millisecond Pulsar: PSR J0437–4715

The paper by Choudhury et al. investigates PSR J0437–4715, the brightest millisecond pulsar (MSP) observed in X-ray wavelengths, using data collected from the Neutron Star Interior Composition Explorer (NICER). The research employs the X-ray Pulse Simulation and Inference (X-PSI) framework to extract the pulsar's mass, radius, and surface hot region properties. Through Bayesian inference techniques, the paper contributes to our understanding of neutron star characteristics, particularly those related to the pulsar's geometry and the equation of state (EoS) of dense matter in neutron stars.

The core objective of this research is to enhance the modeling accuracy of MSPs using pulse profile modeling (PPM), which relies on characterizing the thermal X-ray emission from heated polar caps. The researchers leverage high-quality NICER data to explore the physical attributes of PSR J0437–4715, also incorporating constraints from precise radio pulsar timing conducted via the Parkes Pulsar Timing Array (PPTA).

Key Findings

1. Mass and Radius Measurement: By integrating radio timing priors and various NICER-specific background models, the mass of PSR J0437–4715 is determined as $M = 1.418 \pm 0.037$ M$_\odot$ and the equatorial radius as $R = 11.36^{+0.95}_{-0.63}$ km. These results imply preferences for softer dense matter EoSs, consistent with gravitational wave constraints from neutron star binary mergers.
2. Hot Spot Geometry: The analysis reveals non-antipodal hot emitting regions, inconsistent with a pure, centered dipole magnetic field configuration. The favored model suggests a complex geometry with dual temperature spots formed through certain overlapping regions, indicating a potential deviation from dipolar surface heating models.
3. Instrumental and Methodological Concerns: Taking advantage of both NICER and XMM-Newton datasets enables improvements in background subtraction and modeling accuracy. This approach emphasizes the necessity to handle variability in astrophysical background sources, like the nearby AGN, which can influence X-ray data interpretation.

Implications

The precise measurements and modeling of PSR J0437–4715’s attributes extend our knowledge of neutron star interiors, addressing fundamental questions concerning highly dense matter EoS. Detecting non-antipodal hot spots directly affects our assumptions about magnetic field configurations in MSPs and, consequently, our understanding of their magnetospheric dynamics.

This paper also underscores the importance of high-fidelity, multi-wavelength observations in MSP research. By demonstrating the utility of integrating a powerful set of observational tools with advanced modeling software, this research paves the way for future studies aimed at unraveling the complexities of neutron star physics.

Speculations and Future Directions

As the paper indicates, continued accumulation of photons from NICER will potentially narrow uncertainties on PSR J0437–4715’s parameters even further. Moreover, expanding this framework to other MSPs will provide broader insights into the universal properties of neutron star interiors and dynamics. There is also inherent value in exploring different atmospheric composition models for neutron stars to assess potential variations in inferred properties.

In sum, this research represents a step forward in the precision measurement of neutron star properties by employing advanced computational astrophysics techniques. Efforts building upon this investigation might address outstanding questions related to the nature of dense matter, contributing vital insights applicable across the fields of astrophysics and nuclear physics.