[Paper Review] A NICER View of the Massive Pulsar PSR J0740+6620 Informed by Radio Timing and XMM-Newton Spectroscopy

Riley, Thomas E., Watts, Anna L.+28 more|UvA-DARE (University of Amsterdam)|May 14, 2021

Pulsars and Gravitational Waves ResearchPhysics and Astronomy166 references958 citations

TL;DR

The paper performs Bayesian inference of the radius and mass of the massive millisecond pulsar PSR J0740+6620 by combining NICER X-ray pulse-profile modeling with informative radio timing priors and XMM-Newton spectroscopy. It yields constraints on radius and mass conditional on two hot surface regions and uses open-source software for reproducibility.

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.

Motivation & Objective

• Infer the equatorial radius and mass of PSR J0740+6620 from NICER X-ray pulse-profile modeling conditioned on informative radio timing priors.
• Incorporate XMM-Newton EPIC spectroscopy to inform the X-ray likelihood function.
• Explore surface hot-region models and assess the impact on radius inference.
• Ensure priors for mass, distance, and inclination reflect joint radio timing constraints while allowing for EOS-relevant coverage.
• Provide data and software to enable reproduction of the analysis and EOS implications.

Proposed method

• Bayesian estimation of neutron star radius and mass conditional on NICER XTI pulse-profile data.
• Joint priors on mass, distance, and inclination derived from NANOGrav × CHIME/Pulsar radio timing measurements (Fonseca et al. 2021a).
• Relativistic ray-tracing of X-ray photons from oblate, rotating neutron-star surfaces using X-PSI v0.7.
• Two hot-region surface models (non-overlapping spherical caps with hydrogen atmosphere) to describe surface emission.
• Use XMM-Newton EPIC spectroscopy to inform the X-ray likelihood alongside NICER data.
• Marginal likelihood interpretation: mass-radius posterior can be used to construct a marginal likelihood for EOS analyses.

Experimental results

Research questions

• RQ1 What are the mass and equatorial radius of PSR J0740+6620 given NICER pulse-profile data conditioned on informative radio timing priors?
• RQ2 How do surface hot-region configurations affect the inferred radius under the NICER XTI data?
• RQ3 What is the role of XMM-Newton spectroscopy in constraining the X-ray likelihood and, consequently, the mass-radius inference?
• RQ4 How can the mass-radius posterior be leveraged for EOS inferences in a companion analysis?
• RQ5 How sensitive are the results to the adopted priors on distance, mass, and inclination and to the treatment of dispersion-measure variations?

Key findings

• Equatorial radius and mass constrained to R = 12.39 km with -0.98/+1.30 (16th–84th percentiles) and M = 2.072 M⊙ with -0.066/+0.067 (16th–84th percentiles).
• Temperature and size of two non-overlapping hot regions on the surface are inferred with a posteriori uniform effective temperature for each region (log10(T[K]) = 5.99 with -0.06/+0.05).
• Radius inference is conditioned on informative radio timing priors for mass, distance, and orbital inclination, yielding tighter constraints than NICER data alone.
• All analysis software is open-source and data, models, and samples are publicly available, including analysis notebooks and Python modules.
• The marginal likelihood of mass and equatorial radius is recoverable from posterior samples for EOS posterior analyses in a companion study.