A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy (2105.06980v3)

Abstract: We report on Bayesian estimation of the radius, mass, and hot surface regions of the massive millisecond pulsar PSR J0740$+$6620, conditional on pulse-profile modeling of Neutron Star Interior Composition Explorer X-ray Timing Instrument (NICER XTI) event data. We condition on informative pulsar mass, distance, and orbital inclination priors derived from the joint NANOGrav and CHIME/Pulsar wideband radio timing measurements of arXiv:2104.00880. We use XMM European Photon Imaging Camera spectroscopic event data to inform our X-ray likelihood function. The prior support of the pulsar radius is truncated at 16 km to ensure coverage of current dense matter models. We assume conservative priors on instrument calibration uncertainty. We constrain the equatorial radius and mass of PSR J0740$+$6620 to be $12.39_{-0.98}^{+1.30}$ km and $2.072_{-0.066}^{+0.067}$ M${\odot}$ respectively, each reported as the posterior credible interval bounded by the 16% and 84% quantiles, conditional on surface hot regions that are non-overlapping spherical caps of fully-ionized hydrogen atmosphere with uniform effective temperature; a posteriori, the temperature is $\log{10}(T$ [K]$)=5.99_{-0.06}^{+0.05}$ for each hot region. All software for the X-ray modeling framework is open-source and all data, model, and sample information is publicly available, including analysis notebooks and model modules in the Python language. Our marginal likelihood function of mass and equatorial radius is proportional to the marginal joint posterior density of those parameters (within the prior support) and can thus be computed from the posterior samples.

• The paper presents a multivariate Bayesian approach combining NICER, radio timing, and XMM-Newton spectroscopy for precise pulsar parameter estimation.
• It determines PSR J0740+6620’s mass at approximately 2.07 M☉ and radius around 12.39 km, thereby narrowing the range of viable dense matter EOS models.
• The findings critically inform neutron star EOS research by excluding softer EOS configurations and hinting at exotic matter states under supranuclear densities.

An Overview of the NICER View of the Massive Pulsar PSR J0740+6620

The paper "A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-NEWTON Spectroscopy" by Riley et al. explores the detailed parameters of the pulsar PSR J0740+6620, utilizing the X-ray data obtained from NASA's Neutron Star Interior Composition Explorer (NICER) and insights from radio timing and XMM-NEWTON spectroscopy. This work provides critical measurements of the neutron star's mass, radius, and other surface characteristics, contributing essential data to the understanding of the equation of state (EOS) for dense matter.

Methodological Framework

The authors employ Bayesian statistical methods for estimating the pulsar's parameters, which include its radius, mass, and the configuration of its hot surface regions. The NICER X-ray Timing Instrument data is employed, augmented by accurate radio timing information from NANOGrav and CHIME/Pulsar observations. The resulting estimates are further informed by XMM-NEWTON spectroscopy. This multivariate approach ensures that various observational data types contribute to an integrated model, improving the reliability of the inferred stellar parameters.

Results and Numerical Findings

The paper yields constraints on the equatorial radius and mass of PSR J0740+6620, approximately determining them to be 12.39 km (+1.30, -0.98 km) and 2.072 solar masses (+0.067, -0.066 M_\odot), respectively. These figures offer valuable data for EOS models, primarily by limiting the mass-radius parameter space available to different theoretical EOS configurations. Based on these measurements, the research suggests significant insights into the dense matter EOS, particularly regarding the composition and interactions at supranuclear densities.

Implications for Dense Matter EOS

The results have profound implications for the theoretical modeling of neutron stars. The estimates of PSR J0740+6620's radius at such a high mass position it as a critical point of reference for testing hypotheses about the EOS, especially for models predicting the presence of quark matter or other exotic states at these densities. The constraints provided might exclude particularly soft EOS models unable to support such high-mass neutron stars, while also providing pathways for further exploration into the onset of new physics in neutron stars' cores.

Future Directions and Theoretical Implications

The work sets the stage for future studies that could incorporate additional observations and more sophisticated modeling techniques, potentially involving theoretical developments in the understanding of nuclear interactions at extreme densities. Further, by correlating mass and radius measurements with other astronomical phenomena, researchers may fine-tune models that describe neutron stars and the matter composing them.

In summary, Riley et al. have extended the understanding of high-density EOS through the precise parameter extraction of the pulsar PSR J0740+6620. The fusion of NICER and XMM-NEWTON data with advanced computational modeling presents a template for future efforts in the astrophysical paper of neutron stars.