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 (EoS) of this dense matter is to measure both a star's equatorial circumferential radius Re 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 km and (68%). The independent analysis reported in the companion paper by Riley et al. explores different emitting spot models, but finds spot shapes and locations and estimates of Re and M that are consistent with those found in this work. We show that our measurements of Re and M for PSR J0030+0451 improve the astrophysical constraints on the EoS of cold, catalyzed matter above nuclear saturation density.
The increasing richness of data related to cold dense matter, from laboratory experiments to neutron-star observations, requires a framework for constraining the properties of such matter that makes ...use of all relevant information. Here, we present a rigorous but practical Bayesian approach that can include diverse evidence, such as nuclear data and the inferred masses, radii, tidal deformabilities, moments of inertia, and gravitational binding energies of neutron stars. We emphasize that the full posterior probability distributions of measurements should be used rather than, as is common, imposing a cut on the maximum mass or other quantities. Our method can be used with any parameterization of the equation of state (EOS). We use both a spectral parameterization and a piecewise polytropic parameterization with variable transition densities to illustrate the implications of current measurements and show how future measurements in many domains could improve our understanding of cold catalyzed matter. We find that different types of measurements will play distinct roles in constraining the EOS in different density ranges. For example, better symmetry energy measurements will have a major influence on our understanding of matter somewhat below nuclear saturation density but little influence above that density. In contrast, precise radius measurements or multiple tidal deformability measurements of the quality of those from GW170817 or better will improve our knowledge of the EOS over a broader density range.
Abstract
PSR J0740+6620 has a gravitational mass of 2.08 ± 0.07
M
⊙
, 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
−
1.5
+
2.6
km (68%). We apply our measurement, combined with the previous NICER mass and radius measurement of PSR J0030+0451, the masses of two other ∼2
M
⊙
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 ∼1.5–5 times nuclear saturation density. For a given framework, when all measurements are included, the radius of a 1.4
M
⊙
neutron star is known to ±4% (68% credibility) and the radius of a 2.08
M
⊙
neutron star is known to ±5%. The full radius range that spans the ±1
σ
credible intervals of all the radius estimates in the three frameworks is 12.45 ± 0.65 km for a 1.4
M
⊙
neutron star and 12.35 ± 0.75 km for a 2.08
M
⊙
neutron star.
The Neutron Star Interior Composition Explorer (NICER) on the International Space Station (ISS) observed strong photospheric expansion of the neutron star in 4U 1820-30 during a Type I X-ray burst. A ...thermonuclear helium flash in the star's envelope powered a burst that reached the Eddington limit. Radiation pressure pushed the photosphere out to ∼200 km, while the blackbody temperature dropped to 0.45 keV. Previous observations of similar bursts were performed with instruments that are sensitive only above 3 keV, and the burst signal was weak at low temperatures. NICER's 0.2-12 keV passband enables the first complete detailed observation of strong expansion bursts. The strong expansion lasted only 0.6 s, and was followed by moderate expansion with a 20 km apparent radius, before the photosphere finally settled back down at 3 s after the burst onset. In addition to thermal emission from the neutron star, the NICER spectra reveal a second component that is well fit by optically thick Comptonization. During the strong expansion, this component is six times brighter than prior to the burst, and it accounts for 71% of the flux. In the moderate expansion phase, the Comptonization flux drops, while the thermal component brightens, and the total flux remains constant at the Eddington limit. We speculate that the thermal emission is reprocessed in the accretion environment to form the Comptonization component, and that changes in the covering fraction of the star explain the evolution of the relative contributions to the total flux.
Accretion disks around neutron stars regularly undergo sudden strong irradiation by Type-I X-ray bursts powered by unstable thermonuclear burning on the stellar surface. We investigate the impact on ...the disk during one of the first X-ray burst observations with the Neutron Star Interior Composition Explorer (NICER) on the International Space Station. The burst is seen from Aql X-1 during the hard spectral state. In addition to thermal emission from the neutron star, the burst spectrum exhibits an excess of soft X-ray photons below 1 keV, where NICER's sensitivity peaks. We interpret the excess as a combination of reprocessing by the strongly photoionized disk and enhancement of the pre-burst persistent flux, possibly due to Poynting-Robertson drag or coronal reprocessing. This is the first such detection for a short sub-Eddington burst. As these bursts are observed frequently, NICER will be able to study how X-ray bursts affect the disk and corona for a range of accreting neutron star systems and disk states.
The magnetic field of the classical T Tauri star V2129 Oph can be modelled approximately by superposing slightly tilted dipole and octupole moments, with polar magnetic field strengths of 0.35 and ...1.2 kG, respectively, as observed by Donati et al. Here we construct a numerical model of V2129 Oph incorporating this result and simulate accretion on to the star using a three-dimensional magnetohydrodynamic code. Simulations show that the disc is truncated by the dipole component and matter flows towards the star in two funnel streams. Closer to the star, the flow is redirected by the octupolar component, with some of the matter flowing towards the high-latitude poles, and the rest into the octupolar belts. The shape and position of the spots differ from those in a pure dipole case, where crescent-shaped spots are observed at the intermediate latitudes.
Simulations show that if the disc is truncated at the distance of r≈ 6.2R
★ which is comparable with the corotation radius, r
cor≈ 6.8 R
★, then the high-latitude polar spot dominates, but the accretion rate obtained from the simulations (and from the accompanying theoretical calculations) is about an order of magnitude lower than the observed one. The accretion rate matches the observed one if the disc is disrupted much closer to the star, at 3.4R
★. However, in that case the octupolar belt spots strongly dominate. In the intermediate case of r≈ 4.3R
★, the polar spots are sufficiently bright, and the accretion rate is within the error bar of the observed accretion rate, and this model can explain the observations. However, an even better match has been obtained in experiments with a dipole field twice as strong compared with one suggested by Donati et al.
The torque on the star from the disc-magnetosphere interaction is small, and the time-scale of spin evolution, 2 × 107-6 × 108 yr is longer than the 2 × 106 yr age of V2129 Oph. This means that V2129 Oph probably lost most of its angular momentum in the early stages of its evolution, possibly, during the stage when it was fully convective, and had a stronger magnetic field. The propeller mechanism could also be responsible for the rapid spin-down.
The external magnetic flux of the star is strongly influenced by the disc: the field lines connecting the disc and the star inflate and form magnetic towers above and below the disc. The potential (vacuum) approximation is still valid inside the Alfvén (magnetospheric) surface where the magnetic stress dominates over the matter stress.
Sting like a bee: Nanoarrays of infectious virus particles, encoded with EGFP, are patterned by dip‐pen nanolithography and exposed to a solution of cells. Upon infection, infected cells produce the ...EGFP protein, generating a green fluorescence signal that allows one to monitor the cellular infection process in real time (as seen in the optical image). These results suggest that antibody‐immobilized virus particles retain their biological activity. Scale bar: 35μm.
Motivated by the report by Cottam et al. of iron resonance scattering lines in the spectra of thermonuclear bursts from EXO 0748-676, we have investigated the information about neutron star structure ...and the geometry of the emission region that can be obtained by analyzing the profiles of atomic lines formed at the surface of the star. We have calculated the detailed profiles of such lines, taking into account the star's spin and the full effects of special and general relativity, including light bending and frame dragging. We discuss the line shapes produced by rotational Doppler broadening and magnetic splitting of atomic lines for the spin rates and magnetic fields expected in neutron stars in low-mass X-ray binary systems. We show that narrow lines are possible even for rapidly spinning stars if the emission region or the line of sight are close to the spin axis. For most neutron stars in low-mass systems, magnetic splitting is too small to obscure the effects of special and general relativity. We show that the ratio of the star's mass to its equatorial radius can be determined to within 5% using atomic line profiles, even if the lines are broad and skewed. This is the precision required to constrain strongly the equation of state of neutron star matter. We show further that if the radius and latitude of emission are known to 65%-10% accuracy, then frame dragging has a potentially detectable effect on the profiles of atomic lines formed at the stellar surface.
Immobile but active. Unmodified antibodies can be immobilized in an active state on arrays of patterned features coupled with metal‐ions by using dip‐pen nanolithography and microcontact printing. ...Their immunoreactivity was determined by atomic force and fluorescence microscopy after the arrays had been exposed to their corresponding protein or virus antigens. An example of a functional antibody array used to capture the influenza virus is shown in the AFM image.
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