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We report on Bayesian estimation of the radius, mass, and hot surface regionsof the massive millisecond pulsar PSR J0740$+$6620, conditional onpulse-profile modeling of Neutron Star Interior Composition Explorer X-rayTiming Instrument (NICER XTI) event data. We condition on informative pulsarmass, distance, and orbital inclination priors derived from the joint NANOGravand CHIME/Pulsar wideband radio timing measurements of arXiv:2104.00880. We useXMM European Photon Imaging Camera spectroscopic event data to inform our X-raylikelihood function. The prior support of the pulsar radius is truncated at 16km to ensure coverage of current dense matter models. We assume conservativepriors on instrument calibration uncertainty. We constrain the equatorialradius and mass of PSR J0740$+$6620 to be $12.39_{-0.98}^{+1.30}$ km and$2.072_{-0.066}^{+0.067}$ M$_{\odot}$ respectively, each reported as theposterior credible interval bounded by the 16% and 84% quantiles, conditionalon surface hot regions that are non-overlapping spherical caps of fully-ionizedhydrogen atmosphere with uniform effective temperature; a posteriori, thetemperature 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 analysisnotebooks and model modules in the Python language. Our marginal likelihoodfunction of mass and equatorial radius is proportional to the marginal jointposterior density of those parameters (within the prior support) and can thusbe computed from the posterior samples.

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