The Radius of PSR J0740+6620 from NICER and XMM-Newton Data (2105.06979v1)

Abstract: PSR J0740$+$6620 has a gravitational mass of $2.08\pm 0.07~M_\odot$, which is the highest reliably determined mass of any neutron star. As a result, a measurement of its radius will provide unique insight into the properties of neutron star core matter at high densities. Here we report a radius measurement based on fits of rotating hot spot patterns to Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) X-ray observations. We find that the equatorial circumferential radius of PSR J0740$+$6620 is $13.7^{+2.6}_{-1.5}$ km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030$+$0451, the masses of two other $\sim 2~M_\odot$ pulsars, and the tidal deformability constraints from two gravitational wave events, to three different frameworks for equation of state modeling, and find consistent results at $\sim 1.5-3$ times nuclear saturation density. For a given framework, when all measurements are included the radius of a $1.4~M_\odot$ neutron star is known to $\pm 4$% (68% credibility) and the radius of a $2.08~M_\odot$ neutron star is known to $\pm 5$%. The full radius range that spans the $\pm 1\sigma$ credible intervals of all the radius estimates in the three frameworks is $12.45\pm 0.65$ km for a $1.4~M_\odot$ neutron star and $12.35\pm 0.75$ km for a $2.08~M_\odot$ neutron star.

• The paper reports an equatorial radius of 13.7 km for PSR J0740+6620 with 68% credibility, advancing our understanding of dense matter.
• It employs robust statistical methods combining NICER and XMM-Newton X-ray data to model the star's atmospheric and gravitational lensing effects.
• The findings narrow the range of viable EOS models by indicating consistent radii around 12.5 km for both 1.4 M⊙ and 2.08 M⊙ neutron stars.

The Radius of PSR J0740+6620: Insights from NICER and XMM-Newton Observations

The measurement of the radius of PSR J0740+6620 offers a significant advancement in understanding the properties of neutron star core matter at high densities. PSR J0740+6620, observed to possess a gravitational mass of $2.08\pm 0.07~M_\odot$, represents the heaviest neutron star with a reliably measured mass. Using the Neutron Star Interior Composition Explorer (NICER) and X-ray Multi-Mirror (XMM-Newton) observatories, the authors report an equatorial circumferential radius of $13.7^{+2.6}_{-1.5}$ km, with 68% credibility. This measurement, in conjunction with existing data from PSR J0030+0451, high-mass pulsars, and tidal deformability constraints from gravitational wave events, offers a more precise picture of the equation of state (EOS) of neutron star matter at densities ranging from 1.5 to 3 times nuclear saturation density.

The authors apply robust statistical techniques across three different frameworks for EOS modeling to integrate multifaceted observational data. The combination of NICER and XMM-Newton data, particularly the modulation fraction observed in NICER and the total flux recorded by XMM-Newton, places stringent constraints on the stellar compactness. The authors’ analysis yields a $\pm 1\sigma$ range for the radius of a $1.4~M_\odot$ neutron star as $12.45 \pm 0.65$ km and for a $2.08~M_\odot$ star as $12.35 \pm 0.75$ km.

The data analysis involves sophisticated modeling of the X-ray emission from the neutron star's surface, considering factors such as the atmospheric composition and gravitational lensing effects. The researchers assume a hydrogen atmosphere with potential partial ionization, given the expected accretion history and magnetic field strength ($\sim 3 \times 10^8$ G) of the neutron star.

The results have far-reaching implications for our understanding of the EOS of dense matter, essentially narrowing the range of permissible models. Incorporation of these findings into EOS models suggests a consistent radius estimate around 12.5 km for typical and high-mass neutron stars, reducing the prior uncertainties significantly. Importantly, the inferred radius rules out extremely small or large radii, thus refining the field of physically plausible EOS models.

In theoretical terms, the rigorous integration of diverse datasets underscores the critical role that observations across different spectra and phenomena play in illuminating the nature of neutron star cores. Practically, this radii measurement enhances predictive accuracy regarding gravitational wave emissions from mixed compact object binaries and informs searches for similarly extreme stellar objects.

Future prospects involve narrowing uncertainty bounds further by continued observation with NICER and developing analytic models that assimilate novel observational evidence, including additional gravitational wave detections and sensitive X-ray pulsation data. Overall, this paper not only contributes vital specifics about PSR J0740+6620's structure but also sets new boundaries for theoretical models of ultradense matter.