Neutron Star Research Articles

Formation of lower mass-gap black hole–neutron star binary mergers through super-Eddington stable mass transfer

ABSTRACT Super-Eddington accretion of neutron stars (NSs) has been suggested both observationally and theoretically. In this paper, we propose that NSs in close-orbit binary systems with companions of helium (He) stars, most of which systems form after the common-envelope phase, could experience super-Eddington stable Case BB/BC mass transfer (MT), and can sometimes undergo accretion-induced collapse (AIC), resulting in the formation of lower mass-gap black holes (mgBHs). Our detailed binary evolution simulations reveal that AIC events tend to happen if the primary NSs have an initial mass ${\gtrsim} 1.7\, {\rm M}_\odot$ with a critical accretion rate of ≳300 times the Eddington limit. These mgBHs would have a mass nearly equal to or slightly higher than the NS maximum mass. The remnant mgBH–NS binaries after the core collapses of He stars are potential progenitors of gravitational-wave (GW) sources. Multimessenger observations between GW and kilonova signals from a population of high-mass binary NS and mgBH–NS mergers formed through super-Eddington stable MT are helpful in constraining the maximum mass and equation of state of NSs.

Independent evidence in multi-messenger astrophysics

In this paper I discuss the first “multi-messenger” observations of a binary neutron star merger and kilonova. These observations, touted as “revolutionary,” included both gravitational-wave and electromagnetic observations of a single source. I draw on analogies between astrophysics and historical sciences (e.g., paleontology) to explain the significance of this for (gravitational-wave) astrophysics. In particular, I argue that having independent lines of evidence about a target system enables the use of argumentative strategies—the “Sherlock Holmes” method and consilience—that help overcome the key challenges astrophysics faces as an observational and historical science.

Generation of photoionized plasmas in the laboratory of relevance to accretion-powered x-ray sources using keV line radiation

We describe laboratory experiments to generate x-ray photoionized plasmas of relevance to accretion-powered x-ray sources such as neutron star binaries and quasars, with significant improvements over previous work. A key quantity is referenced, namely the photoionization parameter, defined as ξ=4πF/ne where F is the x-ray flux and ne the electron density. This is normally meaningful in an astrophysical steady-state context, but is also commonly used in the literature as a figure of merit for laboratory experiments that are, of necessity, time-dependent. We demonstrate emission-weighted values of ξ>50 erg-cm s−1 using laser-plasma x-ray sources, with higher results at the centre of the plasma which are in the regime of interest for several astrophysical scenarios. Comparisons of laboratory experiments with astrophysical codes are always limited, principally by the many orders of magnitude differences in time and spatial scales, but also other plasma parameters. However useful checks on performance can often be made for a limited range of parameters. For example, we show that our use of a keV line source, rather than the quasi-blackbody radiation fields normally employed in such experiments, has allowed the generation of the ratio of inner-shell to outer-shell photoionization expected from a blackbody source with ∼keV spectral temperature. We compare calculations from our in-house plasma modelling code with those from Cloudy and find moderately good agreement for the time evolution of both electron temperature and average ionisation. However, a comparison of code predictions for a K-β argon X-ray spectrum with experimental data reveals that our Cloudy simulation overestimates the intensities of more highly ionised argon species. This is not totally surprising as the Cloudy model was generated for a single set of plasma conditions, while the experimental data are spatially integrated.

Quark-hadron pasta phase in neutron stars: The role of medium-dependent surface and curvature tensions

Quark-hadron pasta phase in neutron stars: The role of medium-dependent surface and curvature tensions

Geometric and thermodynamic characterization of binary neutron star accretion discs

Geometric and thermodynamic characterization of binary neutron star accretion discs

Magnetospheric flows in X-ray pulsars – I. Instability at super-Eddington regime of accretion

ABSTRACT Within the magnetospheric radius, the geometry of accretion flow in X-ray pulsars is shaped by a strong magnetic field of a neutron star. Starting at the magnetospheric radius, accretion flow follows field lines and reaches the stellar surface in small regions located close to the magnetic poles of a star. At low mass accretion rates, the dynamics of the flow is determined by gravitational attraction and rotation of the magnetosphere due to the centrifugal force. At the luminosity range close to the Eddington limit and above it, the flow is additionally affected by the radiative force. We construct a model simulating accretion flow dynamics over the magnetosphere, assuming that the flow strictly follows field lines and is affected by gravity, radiative, and centrifugal forces only. The magnetic field of a neutron star is taken to be dominated by the dipole component of arbitrary inclination with respect to the accretion disc plane. We show that accretion flow becomes unstable at high mass accretion rates and tends to fluctuate quasi-periodically with a typical period comparable to the free-fall time from the inner disc radius. The inclination of a magnetic dipole with respect to the disc plane and strong anisotropy of X-ray radiation stabilize the mass accretion rate at the poles of a star, but the surface density of material covering the magnetosphere fluctuates even in this case.

Extracting the hyperon-nucleon interaction via collective flows in heavy-ion collisions

The 2-body hyperon-nucleon interaction softens the equation of state of neutron star matter and leads to the maximum mass of neutron star to be 1.3-1.6M⊙, which deviates from the recent observation with the masses of PSR J0348+0432 (2.01±0.04M⊙) and J0740+6620 (2.14−0.09+0.10M⊙). One attempt is to introduce the more repulsive hyperon-nucleon interaction above the saturation density. A phenomenological Λ potential by fitting the 3-body ΛNN results of chiral effective field theory is implemented into the quantum molecular dynamics transport model to solve the ‘hyperon puzzle’ in neutron stars. It is found that the directed and elliptic flows are sensitive to the high-density hyperon-nucleon potentials in collisions of 197Au + 197Au and 124Sn + 124Sn. The influence of the phenomenological potential on the Λ yields, rapidity distributions and transverse momentum spectra is negligible in comparison with the ones calculated by the well-known relativistic mean-field model. The inclusion of the ΛN, ΣN and ΞN potentials leads to the reduction of hyperon production in the midrapidity region. The Λ directed flows and the slope in the midrapidity region are enhanced with the phenomenological potential and more consistent with the STAR data in collisions of 197Au + 197Au at sNN=3 GeV.

Radio spectra of pulsars fitted with the spectral distribution function of the emission from their current sheet

ABSTRACT In their catalogue of pulsars’ radio spectra, Swainston et al. distinguish between five different forms of these spectra: those that can be fitted with (i) a simple power law, (ii) a broken power law, (iii) a low-frequency turn-over, (iv) a high-frequency turn-over or (v) a double turn-over spectrum. Here, we choose two examples from each of these categories and fit them with the spectral distribution function of the caustics that are generated by the superluminally moving current sheet in the magnetosphere of a non-aligned neutron star. In contrast to the prevailing view that the curved features of pulsars’ radio spectra arise from the absorption of the observed radiation in high-density environments, our results imply that these features are intrinsic to the emission mechanism. We find that all observed features of pulsar spectra (including those that are normally fitted with simple or broken power laws) can be described by a single spectral distribution function and regarded as manifestations of a single emission mechanism. From the results of an earlier analysis of the emission from a pulsar’s current sheet and the values of the fit parameters for each spectrum, we also determine the physical characteristics of the central neutron star of each considered example and its magnetosphere.

Neutron stars as dark matter detectors

Neutron stars as dark matter detectors

Bayesian investigation of the neutron star equation of state vs gravity degeneracy

Bayesian investigation of the neutron star equation of state vs gravity degeneracy

Gravitational-wave imprints of nonconvex dynamics in binary neutron star mergers

Gravitational-wave imprints of nonconvex dynamics in binary neutron star mergers

Neutrino signals from neutron star implosions to black holes

Neutrino signals from neutron star implosions to black holes

Elastic and breaking properties of epitaxial face‐centered crystals in neutron star crusts and white dwarf cores

Elastic and breaking properties of epitaxial face‐centered crystals in neutron star crusts and white dwarf cores

Decay Characteristics of Neutron Excess Potassium Nuclei

Bevelacqua Resources, 7531 Flint Crossing Circle SE, Owens Cross Roads, AL 35762 USA bevelresou@aol.com In neutron star mergers, neutron excess nuclei and the r-process are important factors governing the production of heavy nuclear systems. A single-particle model evaluation of potassium nuclei suggests that the heaviest Z = 19 nucleus will have mass 69 with filling of the 1g9/2 neutron shell. A = 55 – 69 potassium isotopes have limited experimental half-life data, but the model predicts beta decay half-lives in the range of 0.315 – 9.23 ms. Based on previous calculations for Z = 9 - 18, 20, 26, and 30 systems and comparisons to the 55K – 59K, 61K, and 63K calculations, summarized in the Japanese Nuclear Data Compilation, the single-particle model results likely overestimate the half-lives of A = 55 – 69 neutron excess potassium nuclei. KEYWORDS: Nucleosynthesis, Neutron Excess Potassium Nuclei, Beta Decay, Nuclear Structure

Neutron Star Evolution in f(Q) Modified Gravity Framework

The current study reports the evolution of a Neutron Star in f ( Q ) modified gravity framework, where f ( Q ) is chosen in the form f ( Q ) = Q + aQ , where the non-metricity scalar Q is taken as Q = 6 H 2 and a is constant. H is known as the Hubble parameter. Considering the modified Friedmann equation, we reformulated the density and pressure for the f ( Q ) gravity background. In this modified gravity framework, we derived pressure and density as a function of radius r of the neutron star. The reconstructed density happens to be a monotone increasing function of radius r. This can be interpreted as the accretion of Dark Energy around the Neutron star. To further consolidate the results, we reconstruct the pressure as well, and the behavior of the pressure supports the Dark Energy accretion. The behaviors of the EOS and the model parameters have also been studied in this f ( Q ) setting of Neutron Star.

Pre-supernova evolution and final fate of stellar mergers and accretors of binary mass transfer

The majority of massive stars are expected to exchange mass or merge with a companion during their lives. This immediately implies that most supernovae (SNe) are from such post-mass-exchange objects. Here, we explore how mass accretion and merging affect the pre-SN structures of stars and their final fates. To this end, we modelled these complex processes by rapid mass accretion onto stars of different evolutionary stages and followed their evolution up to iron core collapse. We used the stellar evolution code and inferred the outcome of core-collapse using a neutrino-driven SN model. Our models cover initial masses from $11$ to $70\ and the accreted mass ranges from 10--200<!PCT!> of the initial mass. All models are non-rotating and for solar metallicity. The rapid accretion model offers a systematic way to approach the landscape of mass accretion and stellar mergers. It is naturally limited in scope and serves as a clean zeroth order baseline for these processes. We find that mass accretion, in particular onto post-main-sequence (post-MS) stars, can lead to a long-lived blue supergiant (BSG) phase during which stars burn helium in their cores. In comparison to genuine single stars, post-MS accretors have small core-to-total mass ratios, regardless of whether they end their lives as BSGs or cool supergiants (CSGs), and they can have genuinely different pre-SN core structures. As in single and binary-stripped stars, we find black-hole (BH) formation for the same characteristic CO core masses $M_ CO $ of $ and $ In models with the largest mass accretion, the BH formation landscape as a function of $M_ CO $ is shifted by about $0.5\ to lower masses such accretors are more difficult to explode. We find a tight relation between our neutron-star (NS) masses and the central entropy of the pre-SN models in all accretors and single stars, suggesting a universal relation that is independent of the evolutionary history of stars. Post-MS accretors explode both as BSGs and CSGs, and we show how to understand their pre-SN locations in the Hertzsprung--Russell (HR) diagram. Accretors exploding as CSGs can have much higher envelope masses than single stars. Some BSGs that avoid the luminous-blue-variable (LBV) regime in the HR diagram are predicted to collapse into BHs of up to $50\ while others explode in SNe and eject up to $40\ greatly exceeding ejecta masses from single stars. Both the BH and SN ejecta masses increase to about $80\ in our models when allowing for multiple mergers in initial triple-star systems, and they can be even higher at lower metallicities. Such high BH masses may fall into the pair-instability-SN mass gap and could help explain binary BH mergers involving very massive BHs as observed in GW190521. We further find that some of the BSG models explode as LBVs, which may lead to interacting SNe and possibly even superluminous SNe.

Generalized skyrmion crystals with applications to neutron stars

Generalized skyrmion crystals with applications to neutron stars

Polaronic proton and diproton clustering in neutron-rich matter

We show that the strong spin-triplet neutron-proton interaction causes polaronic protons to occur in neutron matter at subnuclear densities and nonzero temperatures. As the neutron density increases, proton spectra exhibit a smooth crossover from a bare impurity to a repulsive polaron branch; this branch coexists with an attractive polaron branch. With the neutron density increased further, the attractive polarons become stable with respect to deuteron formation. For two adjacent protons, we find that the polaron effects and the neutron-mediated attraction are sufficient to induce a bound diproton, which leads possibly to diproton formation in the neutron skin region of neutron-rich nuclei in laboratories as well as in neutron stars.

Spectral Imager of the Solar Atmosphere: The First Extreme-Ultraviolet Solar Integral Field Spectrograph Using Slicers

Particle acceleration, and the thermalisation of energetic particles, are fundamental processes across the universe. Whilst the Sun is an excellent object to study this phenomenon, since it is the most energetic particle accelerator in the Solar System, this phenomenon arises in many other astrophysical objects, such as active galactic nuclei, black holes, neutron stars, gamma ray bursts, solar and stellar coronae, accretion disks and planetary magnetospheres. Observations in the Extreme Ultraviolet (EUV) are essential for these studies but can only be made from space. Current spectrographs operating in the EUV use an entrance slit and cover the required field of view using a scanning mechanism. This results in a relatively slow image cadence in the order of minutes to capture inherently rapid and transient processes, and/or in the spectrograph slit ‘missing the action’. The application of image slicers for EUV integral field spectrographs is therefore revolutionary. The development of this technology will enable the observations of EUV spectra from an entire 2D field of view in seconds, over two orders of magnitude faster than what is currently possible. The Spectral Imaging of the Solar Atmosphere (SISA) instrument is the first integral field spectrograph proposed for observations at ∼180 Å combining the image slicer technology and curved diffraction gratings in a highly efficient and compact layout, while providing important spectroscopic diagnostics for the characterisation of solar coronal and flare plasmas. SISA’s characteristics, main challenges, and the on-going activities to enable the image slicer technology for EUV applications are presented in this paper.

Curvature Quantization based on the Ehrenfest Paradox in the Bohr Atomic Model

Ehrenfest Paradox has been studied in the Bohr Atomic Model as a theoretical procedure for expressing the atomic coordinate curvature in the term of electromagnetic fine structure/coupling constant α = 1/137. The strength of the curvature corresponding to the gravitational aspect depends on the principal quantum number via a new constant 16α2a02=3,0348×1017m−2, which shows that the value of the curvature is quantised. For instance n = 1, the value is 3, 0349 × 1017m−2. The curvature value in the Bohr atomic model can be used as a standard to compare how strong the curvatures of all systems are. This procedure can also be generalized to the strong and weak interactions in such a way that the value of the curvature can be represented by their own coupling constants. In the real situation, we can take the case of the atom near supermassive objects such as: blackhole, neutron star, white dwarf, etc. In this case, the atom is in the curved space-time, while the space-time curvature is quantized by the atom. Principally, the idea of the curvature and quantization are correlated. The excitation (the change of the state) of the atom or the atomic nuclei will generate the change of the space-time curvature ΔGμν(α)n that manifests a quantized curvature propagation (curvaton) through the space-time coordinate in the form of quantum stress tensor ΔTμν(q2B)=ℏΔσnΔGμν(α)n. It can be viewed as a unit of moving curvature reduced to a gravitational wave. This theory can be considered to expand the unification between quantum mechanics and gravitation.