Quasi-Radial Modes of Pulsating Neutron Stars: Numerical Results for General-Relativistic Rigidly Rotating Polytropic Models

In this paper we compute general-relativistic polytropic models simulating rigidly rotating, pulsating neutron stars. These relativistic compact objects, with a radius of ~10 km and mass between ~1.4 and 3.2 solar masses, are closely related to pulsars. We emphasize on computing the change in the pulsation eigenfrequencies owing to a rigid rotation, which, in turn, is a decisive issue for studying stability of such objects. In our computations, we keep rotational perturbation terms of up to second order in the angular velocity.

Computing Differentially Rotating Neutron Stars Obeying Realistic Equations of State by Using Hartle’s Perturbation Method

In this paper, we use the well-known Hartle’s perturbation method in order to compute models of differentially rotating neutron stars obeying realistic equations of state. In our numerical treatment, we keep terms up to third order in the angular velocity. We present indicative numerical results for models satisfying a particular differential rotation law. We emphasize on computing the change in mass owing to this differential rotation law.

Third-Order Corrections and Mass-Shedding Limit of Rotating Neutron Stars Computed By a Complex-Plane Strategy

We implement the so-called “complex-plane strategy” for computing general-relativistic polytropic models of uniformly rotating neutron stars. This method manages the problem by performing all numerical integrations, required within the framework of Hartle’s perturbation method, in the complex plane. We give emphasis on computing corrections up to third order in the angular velocity, and the mass-shedding limit. We also compute the angular momentum, moment of inertia, rotational kinetic energy, and gravitational potential energy of the models considered.

A key factor to the spin parameter of uniformly rotating compact stars: crust structure

We study the dimensionless spin parameter j ≡ cJ/（GM2） of different kinds of uniformly rotating compact stars, including traditional neutron stars, hyperonic neutron stars and hybrid stars, based on relativistic mean field theory and the MIT bag model. It is found that j ~ 0.7, which had been suggested in traditional neutron stars, is sustained for hyperonic neutron stars and hybrid stars with M 〉 0.5 MG. Not the interior but rather the crust structure of the stars is a key factor to determine jmax for three kinds of selected compact stars. Furthermore, a universal formula j = 0.63（f/fK） -- 0.42（f/fK）2 ＋ 0.48（f/fK）z is suggested to determine the spin parameter at any rotational frequency f smaller than the Keplerian frequency fK.

The Completeness of Accreting Neutron Star Binary Candidates from the Chinese Space Station Telescope

A neutron star(NS)has many extreme physical conditions,and one may obtain some important information about an NS via accreting neutron star binary(ANSB)systems.The upcoming Chinese Space Station Telescope(CSST)provides an opportunity to search for a large sample of ANSB candidates.Our goal is to check the completeness of the potential ANSB samples from CSST data.In this paper,we generate some ANSBs and normal binaries under the CSST photometric system by binary evolution and binary population synthesis method and use a machine learning method to train a classification model.Although the Precision(94.56%)of our machine learning model is as high as before study,the Recall is only about 63.29%.The Precision/Recall is mainly determined by the mass transfer rate between the NSs and their companions.In addition,we also find that the completeness of ANSB samples from CSST photometric data by the machine learning method also depends on the companion mass and the age of the system.ANSB candidates with a low initial mass companion star(0.1 M_(⊙)to 1 M_(⊙))have a relatively high Precision(94.94%)and high Recall(86.32%),whereas ANSB candidates with a higher initial mass companion star(1.1 M_(⊙)to 3 M_(⊙))have similar Precision(93.88%)and quite low Recall(42.67%).Our results indicate that although the machine learning method may obtain a relatively pure sample of ANSBs,a completeness correction is necessary for one to obtain a complete sample.

Quantum Stabilization of General-Relativistic Variable-Density Degenerate Stars

Research by one of the authors suggested that the critical mass of constant-density neutron stars will be greater than eight solar masses when the majority of their neutrons group into bosons that form a Bose-Einstein condensate, provided the bosons interact with each other and have scattering lengths on the order of a picometer. That analysis was able to use Newtonian theory for the condensate with scattering lengths on this order, but general relativity provides a more fundamental analysis. In this paper, we determine the equilibrium states of a static, spherically-symmetric variable-density mixture of a degenerate gas of noninteracting neutrons and a Bose-Einstein condensate using general relativity. We use a Klein-Gordan Lagrangian density with a Gross-Pitaevskii term for the condensate and an effective field for the neutrons. We show that a new class of compact stars can exist with masses above the Oppenheimer-Volkoff limit, provided the scattering length of the bosons is large enough. These stars have no internal singularities, obey causality, and demonstrate a quantum mechanism consistent with general relativity that could prevent collapsed stars from becoming black holes.

Simultaneous observations of a pair of kilohertz QPOs and a plausible 1860Hz QPO from an accreting neutron star system

We report an indication （3.22 or） of ≈ 1860 Hz quasi-periodic oscillations from a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this will be by far the highest frequency feature observed from an accreting neutron star system, and hence could be very useful in understanding such systems. This plausible timing feature was observed simultaneously with lower （≈ 585 Hz） and upper （≈ 904 Hz） kilohertz quasi-periodic oscillations. The two kilohertz quasi-periodic oscillation frequencies had a ratio of ≈ 1.5, and the frequency of the alleged ≈ 1860 Hz feature was close to the triple and the double values of these frequencies. This can be useful for constraining the models of all the three features. In particular, the ≈ 1860Hz feature could be （1） from a new and heretofore unknown class of quasi-periodic oscillations, or （2） the first observed overtone of lower or upper kilohertz quasi-periodic oscillations. Finally, we note that, although the relatively low significance of the ≈ 1860 Hz feature argues for caution, even a 3.22 σ feature at such a uniquely high frequency should be interesting enough to spur a systematic search in the archival data, as well as to scientifically motivate sufficiently large timing instruments for the next generation X-ray missions.

Simulation of the gravitational wave frequency distribution of neutron star-black hole mergers

LIGO-Virgo has observed the gravitational waves(GWs)from the coalescence of binary black hole(BBH)and binary neutron star(BNS)during O1 and O2,and the ones from NS-BH are expected to be hunted in the operating O3 run.The population properties and mass distribution of NS-BH mergers are poorly understood now,thus researchers simulated their chirp mass(M)distribution by a synthetic model,in which the BHs and NSs were inferred by LIGO-Virgo(O1/O2),and obtained the values in the range of 2.1 M_(⊙)<M<7.3 M_(⊙).In this paper,we further simulate the GW frequency(fGW)distribution of NS-BH mergers by the above-stated synthetic model,with a basic binary system model through the Monte Carlo method.Our results predict that the median with 90%credible intervals is 165+475-64 Hz in the case of Schwarzschild BH when the system just before merger,and this GW frequency is expected to increase several times in the merger stage,which is lying in the frequency band of LIGO-Virgo,i.e.,about 15 Hz to a few kHz.Our results provide an important reference for hunting the NS-BH mergers by the on-going O3 run of LIGO-Virgo.

Machine Learning to Search for Accreting Neutron Star Binary Candidates Using Chinese Space Station Telescope Photometric System

Accreting neutron star binary(ANSB)systems can provide some important information about neutron stars(NSs),especially on the structure and the equation of state of NSs.However,only a few ANSBs are known so far.The upcoming Chinese Space Station Telescope(CSST)provides an opportunity to search for a large number of ANSB candidates.We aim to investigate whether or not a machine learning method may efficiently search for ANSBs based on CSST photometric system.In this paper,we generate some ANSBs and normal binaries under CSST photometric system by binary evolution and binary population synthesis method and use a machine learning method to train a classification model.We consider the classical multi-color disk and the irradiated accretion disk,then compare their effects on the classification results.We find that no matter whether the X-ray reprocessing effect is included or not,the machine learning classification accuracy is always very high,i.e.,higher than 96%.If a significant magnitude difference exists between the accretion disk and the companion of an ANSB,machine learning may not distinguish it from some normal stars such as massive main sequence stars,white dwarf binaries,etc.False classifications of the ANSBs and the normal stars highly overlap in a color–color diagram.Our results indicate that machine learning would be a powerful way to search for potential ANSB candidates from the CSST survey.

The Remnant of GW170817: A Trapped Neutron Star with a Massive Incompressible Superfluid Core

Our bimetric spacetime model of glitching pulsars is applied to the remnant of GW170817. Accordingly, pulsars are born with embryonic incompressible superconducting gluon-quark superfluid cores (SuSu-matter) that are embedded in Minkowski spacetime, whereas the ambient compressible and dissipative media (CDM) are imbedded in curved spacetime. As pulsars cool down, the equilibrium between both spacetime is altered, thereby triggering the well-observed glitch phenomena. Based thereon and assuming all neutron stars (NSs) to be born with the same initial mass of , we argue that the remnant of GW170817 should be a relatively faint NS with a massive central core made of SuSu-matter. The effective mass and radius of the remnant are predicted to be and Rrem=10.764 Km, whereas the mass of the enclosed SuSu-core is . Here, about 1/2Mcore is an energy enhancement triggered by the phase transition of the gluon-quark-plasma from the microscopic into macroscopic scale. The current compactness of the remnant is , but predicted to increase as the CDM and cools down, rendering the remnant an invisible dark energy object, and therefore to an excellent black hole candidate.

Investigating the UV-excess in Star Clusters with N-body Simulations:Predictions for Future CSST Observations

We study the origin of the UV-excess in star clusters by performing N-body simulations of six clusters with N = 10 k and N = 100 k(single stars & binary systems) and metallicities of Z = 0.01, 0.001 and 0.0001, using PETAR. All models initially have a 50% primordial binary fraction. Using Galev NB we convert the simulated data into synthetic spectra and photometry for the China Space Station Telescope(CSST) and Hubble Space Telescope(HST). From the spectral energy distributions we identify three stellar populations that contribute to the UVexcess:(1) second asymptotic giant branch stars, which contribute to the UV flux at early times;(2) naked helium stars and(3) white dwarfs, which are long-term contributors to the FUV spectra. Binary stars consisting of a white dwarf and a main sequence star are cataclysmic variable(CV) candidates. The magnitude distribution of CV candidates is bimodal up to 2 Gyr. The bright CV population is particularly bright in FUV-NUV. The FUV-NUV color of our model clusters is 1–2 mag redder than the UV-excess globular clusters in M87 and in the Milky Way. This discrepancy may be induced by helium enrichment in observed clusters. Our simulations are based on simple stellar evolution;we do not include the effects of variations in helium and light elements or multiple stellar populations. A positive radial color gradient is present in CSST NUV-y for main sequence stars in all models with a color difference of 0.2–0.5 mag, up to 4 half-mass radii. The CSST NUV-g color correlates strongly with HST FUV-NUV for NUV-g > 1 mag, with the linear relation FUV-NUV =(1.09 ± 0.12) ×(NUV-g) +(-1.01 ± 0.22). This allows for conversion of future CSST NUV-g colors into HST FUV-NUV colors, which are sensitive to UV-excess features. We find that CSST will be able to detect UVexcess in Galactic/extragalactic star clusters with ages >200 Myr.

X-Ray Properties of PSR J1811-1925 by Nu STAR

We analyzed the spectral properties and pulse profile of PSR J1811-1925,a pulsar located in the center of composite supernova remnant(SNR)G11.2-0.3,by using high timing resolution archival data from the Nuclear Spectroscopic Telescope Array Mission(NuSTAR).Analysis of archival Chandra data over different regions rules out the SNR shell as the site of the hard X-ray emission while spectral analysis indicates that the NuSTAR photons originate in the pulsar and its nebula.The pulse profile exhibits a broad single peak up to 35 keV.The jointed spectrum by combining NuSTAR and Chandra can be well fitted by a power-law model with a photon index ofΓ=1.58±0.04.The integrated flux of jointed spectrum over 1-10 keV is 3.36×10^(-12)erg cm^(-2)s^(-1).The spectrum of pulsar having photon indexΓ=1.33±0.06 and a 1-10 keV flux of 0.91×10^(-12)erg cm^(-2)s^(-1).We also performed the phase-resolved spectral analysis by splitting the whole pulse-on phase into five phase bins.The photon indices of the bins are all around 1.4,indicating that the photon index does not evolve with the phase.

On the Rotation Properties of a Post-explosion Helium-star Companion in Type Iax Supernovae

Recent studies have suggested that type lax supernovae(SNe lax) are likely to result from a weak deflagration explosion of a Chandrasekhar-mass white dwarf in a binary system with a helium(He)-star companion.Assuming that most SNe Iax are produced from this scenario,in this work we extend our previous work on the threedimensional hydrodynamical simulation of ejecta-companion interaction by taking the orbital and spin velocities of the progenitor system into account.We then follow the post-impact evolution of a surviving He-star companion by using the one-dimensional stellar evolution code MESA.We aim to investigate the post-explosion rotation properties of a He-star companion in SNe Iax.We find that the He-star companion spins down after the impact due to the angular-momentum loss and expansion caused by the mass-stripping and shock heating during the interaction.This leads to the situation where the surface rotational speed of the surviving companion can drop to one-third of its pre-explosion value when it expands to a maximum radius a few years after the impact.Subsequently,the star shrinks and spins up again once the deposited energy is released.This spin-switching feature of the surviving He-star companions of SNe Iax may be useful for the identification of such objects in future observations.

LOCV calculation of the equations of state and properties of rapidly rotating neutron stars

In this paper, we have investigated the structural properties of rotating neutron stars using the numerical RNS code and equations of state which have been calculated within the lowest order constrained variational（LOCV）approach. In order to calculate the equation of state of nuclear matter, we have used UV（14） ＋TNI and AV（18） potentials.We have computed the maximum mass of the neutron star and the corresponding equatorial radius at different angular velocities. We have also computed the structural properties of Keplerian rotating neutron stars for the maximum mass configuration, MK, RK, fK and jmax.

AGB星辐射s-过程核合成模型中子辐照量分布的计算方法和特点

考虑到模型参量随脉冲数的变化,推导出^(13)C辐射燃烧的低质量AGB星s-过程核合成模型中子辐照量分布的计算方法,该方法具有普适性和简洁性.利用该方法,计算了3M_⊙、太阳金属丰度恒星模型的中子辐照量分布.结果表明,若合理假设^(13)C薄层内中子数密度均匀分布,则辐射核合成模型最终的中子辐照量分布趋近于指数分布.对于初始质量一定、金属丰度一定的恒星模型,平均中子辐照量τ_0与每个脉冲的中子辐照量△7存在确定的关系:τ_0=0.434λ(q_1,q_2…q_(m_(max))+1,r_1,r_2…r_(m_(max))+1)△7,式中m_(max)是带挖掘的脉冲总数,比例系数λ(q_1,q_2…q_(m_(max))+1,r_1,r_2…r_(m_(max))+1)可通过对最终中子辐照量分布的指数拟合得到.该式从中子辐照量分布角度定量地将经典模型和辐射s-过程核合成模型统一起来,使经典模型能为恒星模型核合成数值计算提供指导和约束.

AGB星s-过程核合成:中子辐照量渐近分布

根据13C壳层在脉冲间隔期间辐射燃烧释放中子的AGB星s(slow) 过程核合成模型,采用3层简化假设,研究了氦中间壳层的中子辐照量渐近分布。结果表明,中子辐照量分布不再是通常所认为的指数形式,曲线先有一个简短的上升然后再单调下降,但除中子辐照量较低的一个小区域外,分布曲线仍与指数分布非常接近。这与根据恒星模型进行的核合成计算结果一致。

AGB星s-过程核合成区域中子辐照量分布

介绍了AGB(Asymptotic Giant Branch)星s-过程核合成区域中子辐照量分布的最新研究结果。然后将不同AGB星s-过程核合成模型给出的太阳系中子辐照量分布与由观测约束得出的结果进行对比,分析论证了最新研究结果的可靠性。

AGB星s-过程核合成相关问题的研究进展

综述了近年来AGB星核合成理论的研究情况,述及AGB星的结构与s-过程核合成有关的中子辐照量分布、人们比较关注的铅星与非铅星、后AGB星元素丰度分布及与AGB星核合成有关的s+r星。

AGB星s-过程核合成:铁种子核受中子照射的概率

根据3种典型的AGB星s-过程核合成模型的中子辐照量分布,讨论了各恒星模型氦壳层区域内受中子照射的铁种子核数目比率(或铁种子核受中子照射的概率).该值在一定程度上反映了s-过程核合成模型的特点.结果表明,所有概率都随重叠因子r单调增加,r=1时,存在极限值1.在辐射s-过程核合成模型的情形,概率对13C壳层占氦壳层的质量比例q的变化很敏感.

AGB星中子辐照量分布的特点

根据13C辐射燃烧的低质量AGB(asymptotic giant branch)星s-过程核合成模型,考察氦壳层核合成区域中子辐照量分布(简称DNE)的特点.结果表明,辐射s-过程核合成模型的DNE,即ρAGB(τ),与重叠因子r、核合成区域的质量比q、每次中子照射的最大辐照量τmax和脉冲次数m等参数有关.增大各参量的值,都会使具有较大中子辐照量的种子核比例增加,但程度不同.r和m影响较小,q和τmax影响较大,且q与r值相差越多q的影响越显著.选择合适的模型参数,还可以给出太阳系的DNE.根据AGB星s-过程核合成数值计算的结果确定q,r,m的值,通过对多事件模型的结果进行最小二乘法拟合选择τmax的值,最后给出太阳系的DNE.