Showing papers on "Neutron Star Interior Composition Explorer published in 2020"

Constraining the Dense Matter Equation of State with Joint Analysis of NICER and LIGO/Virgo Measurements

Abstract: The Neutron Star Interior Composition Explorer collaboration recentlypublished a joint estimate of the mass and the radius of PSR J0030+0451,derived via X-ray pulse-profile modeling. Raaijmakers et al. exploredthe implications of this measurement for the dense matter equation ofstate (EOS) using two parameterizations of the high-density EOS: apiecewise-polytropic model, and a model based on the speed of sound inneutron stars (NSs). In this work we obtain further constraints on theEOS following this approach, but we also include information about thetidal deformability of NSs from the gravitational wave signal of thecompact binary merger GW170817. We compare the constraints on the EOS tothose set by the recent measurement of a 2.14 M⊙ pulsar,included as a likelihood function approximated by a Gaussian, and find asmall increase in information gain. To show the flexibility of ourmethod, we also explore the possibility that GW170817 was a NS-blackhole merger, which yields weaker constraints on the EOS.

Anisotropic neutron stars modelling: constraints in Krori-Barua spacetime

Abstract: Dense nuclear matter is expected to be anisotropic due to effects such as solidification, superfluidity, strong magnetic fields, hyperons, pion-condensation. Therefore an anisotropic neutron star core seems more realistic than an ideally isotropic one. We model anisotropic neutron stars working in the Krori–Barua (KB) ansatz without preassuming an equation of state. We show that the physics of general KB solutions is encapsulated in the compactness. Imposing physical and stability requirements yields a maximum allowed compactness $$2GM/Rc^2 < 0.71$$ for a KB-spacetime. We further input observational data from numerous pulsars and calculate the boundary density. We focus especially on data from the LIGO/Virgo collaboration as well as recent independent measurements of mass and radius of miilisecond pulsars with white dwarf companions by the Neutron Star Interior Composition Explorer (NICER). For these data the KB-spacetime gives the same boundary density which surprisingly equals the nuclear saturation density within the data precision. Since this value designates the boundary of a neutron core, the KB-spacetime applies naturally to neutron stars. For this boundary condition we calculate a maximum mass of 4.1 solar masses.

Strong-field effects in massive scalar-tensor gravity for slowly spinning neutron stars and application to x-ray pulsar pulse profiles

Abstract: Neutron stars (NSs) in scalar-tensor (ST) theories of gravitation can acquire scalar charges and generate distinct spacetimes from those in general relativity (GR) through the celebrated phenomenon of spontaneous scalarization. Taking on an ST theory with the mass term of the scalar field, we determine the theory parameter space for spontaneous scalarization by investigating the linearized scalar field equation. Then the full numerical solutions for slowly rotating NSs are obtained and studied in great detail. The resulted spacetime is used to calculate test-particle geodesics. The lightlike geodesics are used to construct the profile of x-ray radiation from a pair of hot spots on the surface of scalarized NSs, which potentially can be compared with the data from the Neutron star Interior Composition Explorer (NICER) mission for testing the ST theory.

Anisotropic Neutron Stars Modelling: Constraints in Krori-Barua Spacetime

Abstract: Dense nuclear matter is expected to be anisotropic due to effects such as solidification, superfluidity, strong magnetic fields, hyperons, pion-condesation. Therefore an anisotropic neutron star core seems more realistic than an ideally isotropic one. We model anisotropic neutron stars working in the Krori-Barua (KB) ansatz without preassuming an equation of state. We show that the physics of general KB solutions is encapsulated in the compactness. Imposing physical and stability requirements yields a maximum allowed compactness $2GM/Rc^2 < 0.71$ for a KB-spacetime. We further input observational data from numerous pulsars and calculate the boundary density. We focus especially on data from the LIGO/Virgo collaboration as well as recent independent measurements of mass and radius of miilisecond pulsars with white dwarf companions by the Neutron Star Interior Composition Explorer (NICER). For these data the KB-spacetime gives the same boundary density which surprisingly equals the nuclear saturation density within the data precision. Since this value designates the boundary of a neutron core, the KB-spacetime applies naturally to neutron stars. For this boundary condition we calculate a maximum mass of 4.1 solar masses.

A Numerical Model for the Multi-wavelength Lightcurves of PSR J0030+0451

Abstract: Recent modeling of Neutron Star Interior Composition Explorer(NICER) observations of the millisecond pulsar PSR J0030+0451 suggests that the magnetic field of the pulsar is non-dipolar. We construct a magnetic field configuration where foot points of the open field lines closely resemble the hotspot configuration from NICER observations. Using this magnetic field as input, we perform force-free simulations of the magnetosphere of PSR J0030+0451, showing the three-dimensional structure of its plasma-filled magnetosphere. Making simple and physically motivated assumptions about the emitting regions, we are able to construct the multi-wavelength lightcurves that qualitatively agree with the corresponding observations. The agreement suggests that multipole magnetic structures are the key to modeling this type of pulsars, and can be used to constrain the magnetic inclination angle and the location of radio emission.

Timing of the accreting millisecond pulsar IGR J17591-2342: evidence of spin-down during accretion

Abstract: We report on the phase-coherent timing analysis of the accreting millisecond X-ray pulsar IGR J17591-2342, using Neutron Star Interior Composition Explorer (NICER) data taken during the outburst of the source between 2018 August 15 and 2018 October 17. We obtain an updated orbital solution of the binary system. We investigate the evolution of the neutron star spin frequency during the outburst, reporting a refined estimate of the spin frequency and the first estimate of the spin frequency derivative ($\dot{ u} \sim -7\times 10^{-14}$ Hz s$^{-1}$), confirmed independently from the modelling of the fundamental frequency and its first harmonic. We further investigate the evolution of the X-ray pulse phases adopting a physical model that accounts for the accretion material torque as well as the magnetic threading of the accretion disc in regions where the Keplerian velocity is slower than the magnetosphere velocity. From this analysis we estimate the neutron star magnetic field $B_{eq} = 2.8(3)\times10^{8}$ G. Finally, we investigate the pulse profile dependence on energy finding that the observed behaviour of the pulse fractional amplitude and lags as a function of energy are compatible with a thermal Comptonisation of the soft photons emitted from the neutron star caps.

XMM-Newton and NICER measurement of the rms spectrum of the millihertz quasi-periodic oscillations in the neutron-star low-mass X-ray binary 4U 1636-53

Abstract: We used two XMM-Newton and six Neutron Star Interior Composition Explorer (NICER) observations to investigate the fractional rms amplitude of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636-53 We studied, for the first time, the fractional rms amplitude of the mHz QPOs vs energy in 4U 1636-53 down to 02 keV We find that, as the energy increases from 02 keV to 3 keV, the rms amplitude of the mHz QPOs increases, different from the decreasing trend that has been previously observed above 3 keV This finding has not yet been predicted by any current theoretical model, however, it provides an important observational feature to speculate whether a newly discovered mHz oscillation originates from the marginally stable nuclear burning process on the neutron star surface

Effects of the treatment of the mass quadrupole moment on ray-tracing applications for rapidly-rotating neutron stars

Abstract: The Neutron Star Interior Composition Explorer (NICER) mission has provided a unique opportunity to constrain the equation of state of neutron stars by using the technique of pulse-profile modelling. This technique requires accurate and efficient ray tracing, that in turn requires a robust representation of the spacetime around a neutron star. Several exact and approximate metrics have been proposed, and used, to perform ray tracing around neutron stars, with both moderate and fast rotation. In this paper, we perform a comparison between several of these metrics, when used for ray tracing. We calculate the shape of the neutron star as seen by a distant observer using two different surface formulae, the thermal spectrum and pulse profiles from circular and crescent-shaped hotspots, for four configurations of pulsars with rotation rates ranging from 622 to 1000 Hz, and using both a moderate and a stiff equation of state to include realistic and extreme cases. We find small differences between the metrics for rotation frequencies starting at ~700 Hz that could theoretically be used for constraining the quadrupole moment or the spacetime models. We also determine the practicality of use of each metric in larger-scale applications such as pulse-profile

Inner Workings: Physicists look to a new telescope to understand neutron stars and matter at the extremes.

Abstract: Astronomers ostensibly know plenty about neutron stars: the hot, collapsed remnants of massive stars that have exploded as supernovae. These objects can spin up to hundreds of times a second, generate intense magnetic fields, and send out jets of radiation that sweep the sky like beams from a lighthouse. When two neutron stars collide, the ripples in space-time can be detected by gravitational wave observatories on Earth. Packing more than the mass of the Sun into a ball about 20–25 kilometers across, neutron stars pack in matter at the highest possible density before collapse. “They really are the last gasp of matter, before the event horizon and the nothingness of the black hole,” says physicist Zaven Arzoumanian at the NASA Goddard Space Flight Center in Baltimore, MD. The powerful, spinning magnetic fields of pulsars accelerate charged particles that collide with the surface at hotspots, generating the X-rays that stream out into space. Thanks to NASA’s Neutron star Interior Composition Explorer, Pulsar J0030+0451, visualized here based on data collected, has the most precise measurements of a pulsar’s mass and size. Newly elucidated hotspot shape and location data have challenged conventional views. Image credit: NASA’s Goddard Space Flight Center. And yet, what’s deep inside a neutron star remains an open question. “Understanding what happens to matter at such high densities has long been a puzzle,” says Arzoumanian. Now a small, boxy X-ray telescope mounted on the International Space Station is spilling the inner secrets of these stars. Called the Neutron Star Interior Composition Explorer, or NICER, it can measure the size and mass of neutron stars, revealing their true density. Arzoumanian presented NICER data at the April 2020 meeting of the American Physical Society, which was held online. The early results are promising, showing that the instrument has the potential to capture …

Effects of the treatment of the mass quadrupole moment in ray-tracing applications using Kerr-like metrics for neutron stars

Abstract: The Neutron Star Interior Composition Explorer (NICER) mission has provided a unique opportunity to constrain the equation of state of neutron stars by using the technique of pulse-profile modeling. This technique requires accurate and efficient ray tracing, that in turn requires a robust representation of the spacetime around a neutron star. Several exact and approximate metrics have been proposed, and used, to perform ray tracing around neutron stars, with both slow and moderate rotation. In this paper, we perform a comparison between several of these metrics, when used for ray tracing. We calculate the shape of the neutron star as seen by a distant observer, the thermal spectrum and pulse profiles from circular and crescent-shaped hot spots, for four configurations of pulsars with rotation rates ranging from 622 to 1000 Hz, and using both a moderate and a stiff equation of state to include realistic and extreme cases. We find small differences between the metrics for rotation frequencies starting at ~700 Hz. We determine the practicality of use of each metric in larger-scale applications such as pulse-profile modeling.