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S. M. Morsink

Publications and source records attributed to S. M. Morsink.

9 recordsLinked to original sources
arXiv2026

The Radius of the Neutron Star PSR J0614-3329 from NICER Data

Neutron star radius measurements, particularly using X-ray data collected with the Neutron star Interior Composition Explorer (NICER), have provided invaluable information for models of cold, catalyzed matter at densities above that of nuclear saturation. Here we present an analysis of NICER data on the 318-Hz pulsar PSR J0614-3329, which has a mass ~1.4-1.5 solar masses determined from radio observations. The best-fitting model we explore has three uniform-temperature circular hot spots and yields a symmetric 68% credible range for the equatorial circumferential radius of 9.88-12.77 km. We also explore joint fits to the NICER data and the X-ray Multi-Mirror (XMM-Newton) data on this pulsar, but find that the models that best fit both data sets systematically underpredict the XMM-Newton data when, as is standard, it is assumed that the XMM-Newton background is known from observations of surrounding fields. Finally, we discuss the implications of our results, combined with data on other neutron stars, for the properties of the dense matter in neutron star cores.

astro-ph.HE↗
arXiv2025

The Radius of PSR J0437-4715 from NICER Data

Neutron star Interior Composition Explorer (NICER) data have been used to estimate the masses and radii of the rotation-powered millisecond pulsars PSR J0030$+$0451, PSR J0740$+$6620, PSR J0437$-$4715, PSR J1231$-$1411, and PSR J0614$-$3329, sometimes in joint analyses with X-ray Multi-Mirror (XMM-Newton) data. These measurements provide invaluable information about the properties of cold, catalyzed matter beyond nuclear saturation density. Here we present the results of our modeling of NICER data on PSR J0437$-$4715 using several different models of hot thermal X-ray emitting spots on the stellar surface. For this pulsar, previous Nuclear Spectroscopic Telescope Array (NuSTAR) observations established that there is also a modulated nonthermal component to the emission, but the previously published analysis of NICER data did not model this component. We find that the Bayesian evidence is significantly higher when the modulated nonthermal component is included, and that omission of this component leads to poor fits to the bolometric NICER data and thus risks bias in the resulting radius estimates. Our models, which we pursue to inferential convergence, therefore have modulated nonthermal emission, and our headline model has in addition three uniform-temperature thermally-emitting circular spots. Using this model, the symmetric 68% credible range in the radius is 11.8 km to 15.1 km, which at the independently-measured mass of $M=1.418\pm 0.044~M_\odot$ is consistent with previous reports of the radius of the $\sim 1.4~M_\odot$ pulsar PSR J0030$+$0451. We discuss the implications of this measurement for the equation of state of dense matter.

astro-ph.HE↗
arXiv2021

The Radius of PSR J0740+6620 from NICER and XMM-Newton Data

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 $\pm 1σ$ 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.

astro-ph.HE↗
arXiv2019

A NICER view of PSR J0030+0451: Implications for the dense matter equation of state

Both the mass and radius of the millisecond pulsar PSR J0030+0451 have been inferred via pulse-profile modeling of X-ray data obtained by NASA's NICER mission. In this Letter we study the implications of the mass-radius inference reported for this source by Riley et al. (2019) for the dense matter equation of state (EOS), in the context of prior information from nuclear physics at low densities. Using a Bayesian framework we infer central densities and EOS properties for two choices of high-density extensions: a piecewise-polytropic model and a model based on assumptions of the speed of sound in dense matter. Around nuclear saturation density these extensions are matched to an EOS uncertainty band obtained from calculations based on chiral effective field theory interactions, which provide a realistic description of atomic nuclei as well as empirical nuclear matter properties within uncertainties. We further constrain EOS expectations with input from the current highest measured pulsar mass; together, these constraints offer a narrow Bayesian prior informed by theory as well as laboratory and astrophysical measurements. The NICER mass-radius likelihood function derived by Riley et al. (2019) using pulse-profile modeling is consistent with the highest-density region of this prior. The present relatively large uncertainties on mass and radius for PSR J0030+0451 offer, however, only a weak posterior information gain over the prior. We explore the sensitivity to the inferred geometry of the heated regions that give rise to the pulsed emission, and find a small increase in posterior gain for an alternative (but less preferred) model. Lastly, we investigate the hypothetical scenario of increasing the NICER exposure time for PSR J0030+0451.

astro-ph.HE↗
arXiv2019

PSR J0030+0451 Mass and Radius from NICER Data and Implications for the Properties of Neutron Star Matter

Neutron stars are not only of astrophysical interest, but are also of great interest to nuclear physicists, because their attributes can be used to determine the properties of the dense matter in their cores. One of the most informative approaches for determining the equation of state of this dense matter is to measure both a star's equatorial circumferential radius $R_e$ and its gravitational mass $M$. Here we report estimates of the mass and radius of the isolated 205.53 Hz millisecond pulsar PSR J0030+0451 obtained using a Bayesian inference approach to analyze its energy-dependent thermal X-ray waveform, which was observed using the Neutron Star Interior Composition Explorer (NICER). This approach is thought to be less subject to systematic errors than other approaches for estimating neutron star radii. We explored a variety of emission patterns on the stellar surface. Our best-fit model has three oval, uniform-temperature emitting spots and provides an excellent description of the pulse waveform observed using NICER. The radius and mass estimates given by this model are $R_e = 13.02^{+1.24}_{-1.06}$ km and $M = 1.44^{+0.15}_{-0.14}\ M_\odot$ (68%). The independent analysis reported in the companion paper by Riley et al. (2019) explores different emitting spot models, but finds spot shapes and locations and estimates of $R_e$ and $M$ that are consistent with those found in this work. We show that our measurements of $R_e$ and $M$ for PSR J0030$+$0451 improve the astrophysical constraints on the equation of state of cold, catalyzed matter above nuclear saturation density.

astro-ph.HE↗
arXiv2016

Neutron Star Mass-Radius Constraints using Evolutionary Optimization

The equation of state of cold supra-nuclear-density matter, such as in neutron stars, is an open question in astrophysics. A promising method for constraining the neutron star equation of state is modelling pulse profiles of thermonuclear X-ray burst oscillations from hotspots on accreting neutron stars. The pulse profiles, constructed using spherical and oblate neutron star models, are comparable to what would be observed by a next-generation X-ray timing instrument like ASTROSAT, NICER, or LOFT. In this paper, we showcase the use of an evolutionary optimization algorithm to fit pulse profiles to determine the best-fit masses and radii. By fitting synthetic data, we assess how well the optimization algorithm can recover the input parameters. Multiple Poisson realizations of the synthetic pulse profiles, constructed with 1.6 million counts and no background, were fitted with the Ferret algorithm to analyze both statistical and degeneracy-related uncertainty, and to explore how the goodness-of-fit depends on the input parameters. For the regions of parameter space sampled by our tests, the best-determined parameter is the projected velocity of the spot along the observer's line-of-sight, with an accuracy of $\le3$% compared to the true value and with $\le5$\% statistical uncertainty. The next best-determined are the mass and radius; for a neutron star with a spin frequency of 600\,Hz, the best-fit mass and radius are accurate to $\le5$%, with respective uncertainties of $\le7$% and $\le10$%. The accuracy and precision depend on the observer inclination and spot co-latitude, with values of $\sim1$ % achievable in mass and radius if both the inclination and co-latitude are > 60 degrees.

astro-ph.HE↗
arXiv1995

Covariant double-null dynamics: $(2+2)$-splitting of the Einstein equations

The paper develops a $(2+2)$-imbedding formalism adapted to a double foliation of spacetime by a net of two intersecting families of lightlike hypersurfaces. The formalism is two-dimensionally covariant, and leads to simple, geometrically transparent and tractable expressions for the Einstein field equations and the Einstein-Hilbert action, and it should find a variety of applications. It is applied here to elucidate the structure of the characteristic initial-value problem of general relativity.

gr-qc↗
arXiv1994

Structure of the Spherical Black Hole Interior

The internal structure of a charged spherical black hole is still a topic of debate. In a nonrotating but aspherical gravitational collapse to form a spherical charged black hole, the backscattered gravitational wave tails enter the black hole and are blueshifted at the Cauchy horizon. This has a catastrophic effect if combined with an outflux crossing the Cauchy horizon: a singularity develops at the Cauchy horizon and the effective mass inflates. Recently a numerical study of a massless scalar field in the \RN background suggested that a spacelike singularity may form before the Cauchy horizon forms. We will show that there exists an approximate analytic solution of the scalar field equations which allows the mass inflation singularity at the Cauchy horizon to exist. In particular, we see no evidence that the Cauchy horizon is preceded by a spacelike singularity.

gr-qc↗