Compact Star Systems Research Articles

Significance of Fabry-Perot Cavities for Space Gravitational Wave Antenna DECIGO

DECIGO is a future Japanese project for the detection of gravitational waves in space. To conduct various scientific missions, including the verification of cosmic inflation through the detection of primordial gravitational waves as the main objective, DECIGO is designed to have high sensitivity in the frequency band from 0.1 to 10 Hz, with arms of length 1000 km. Furthermore, the use of the Fabry-Perotcavity in these arms has been established for the DECIGO project. In this paper, we scrutinize the significance of the Fabry-Perot cavity for promoting this project, with a focus on the possibility of observing gravitational waves from cosmic inflation and binary compact star systems as indicators. The results show that using the Fabry-Perot cavity is extremely beneficial for detecting them, and it is anticipated to enable the opening of a new window in gravitational wave astronomy.

Singularity Free Star Model Characterized by Quintessence Field in Quadratic f(Q)$f(Q)$ Gravity

AbstractIn the context of the theory of gravity, a spherically symmetric quintessence DE model in this article is offered. For this reason, has the formula , where Q stands for the non‐metricity scalar and ‘a’ is the coupling constant for modified gravity has been taken into account. It is supposed that the quintessence field defined by the parameter , where , controls the energy‐momentum tensor of the underlying fluid distribution. The relationship between several physical parameters for the selected values of ‘a’, by choosing the metric potentials suggested by the Krori‐Barua [Krori and Barua; J. Phys. A, Math. Gen. 8: 508, 1975] is investigated. Further, a number of analyses in detail to examine the physical validity of the proposed stellar model are carried out. The consequences of the compact star system caused by the connection of matter and geometry are succinctly described. The maximum allowable mass and radius for our present model for different values of ‘a’ have been studied by and curve. One can recover the usual general relativity (GR) standard results for .

Dynamics of the clumps partially disrupted from a planet around a neutron star

ABSTRACT Tidal disruption events are common in the Universe, which may occur in various compact star systems and could account for many astrophysical phenomena. Depending on the separation between the central compact star and its companion, either a full disruption or a partial disruption may occur. The partial disruption of a rocky planet around a neutron star (NS) can produce kilometer-sized clumps, but the main portion of the planet can survive. The dynamical evolution of these clumps is still poorly understood. In this study, the characteristics of partial disruption of a rocky planet in a highly elliptical orbit around a NS is investigated. The periastron of the planet is assumed to be very close to the NS so that it would be partially disrupted by tidal force every time it passes through the periastron. It is found that the fragments generated in the process will change their orbits on a time scale of a few orbital periods due to the combined influence of the NS and the remnant planet, and will finally collide with the central NS. Possible outcomes of the collisions are discussed.

Population Synthesis of Black Hole Binaries with Compact Star Companions

Abstract We perform a systematic study of merging black hole (BH) binaries with compact star (CS) companions, including black hole–white dwarf (BH–WD), black hole–neutron star (BH–NS), and black hole–black hole (BH–BH) systems. Previous studies have shown that mass transfer stability and common envelope evolution can significantly affect the formation of merging BH–CS binaries through isolated binary evolution. With detailed binary evolution simulations, we obtain easy-to-use criteria for the occurrence of the common envelope phase in mass-transferring BH binaries with a nondegenerate donor, and incorporate the criteria into population synthesis calculations. To explore the impact of a possible mass gap between NSs and BHs on the properties of merging BH–CS binary population, we adopt different supernova mechanisms involving the rapid, delayed, and stochastic prescriptions to deal with the compact remnant masses and the natal kicks. Our calculations show that there are ∼105–106 BH–CS binaries in the Milky Way, among which dozens are observable by future space-based gravitational wave detectors. We estimate that the local merger rate density of all BH–CS systems is ∼60–200 Gpc−3 yr−1. While there are no low-mass BHs formed via rapid supernovae, both delayed and stochastic prescriptions predict that ∼100%/∼70%/∼30% of merging BH–WD/BH–NS/BH–BH binaries are likely to have BH components within the mass gap.

Formation and Evolution of Ultraluminous X-Ray Pulsar Binaries to Pulsar–Neutron Star and Pulsar–White Dwarf Systems

Abstract Recent observational and theoretical results have suggested that some of ultraluminous X-ray (ULX) sources may contain neutron star (NS) accretors. However, the formation channel and properties of donor stars of NS ULXs remain uncertain. By adopting the nonconservative and rotation-dependent mass transfer model in the primordial binary evolution, we investigate the way to form pulsar ULXs like observed pulsar ULXs in a systematic way. Our simulation results indicate that pulsar ULXs with Be stars and intermediate or/and high-mass donors match observed apparent luminosities, orbital periods, and observationally indicated donor masses of known pulsar ULXs. ULXs with Be and intermediate donors are main contributors. The route of accretion-induced collapse of WDs has a 4.5% contribution to the NS ULXs, 4.0% of NSs in ULXs are formed through electron-capture supernovae (SNe), and 91.5% of NSs in ULXs are born with core-collapse SNe. We also studied the evolution of pulsar ULXs to double compact star systems. We do not find NS–black hole systems (merging in a Hubble time) that evolved from pulsar ULXs. Pulsar–white dwarf (WD) cases that evolve through pulsar ULXs have significant contributions to the whole NS–WD gravitational wave sources. Contributions of pulsar–WD and pulsar–NS cases that experienced pulsar ULXs are ∼40% and 11% among all LISA NS–WD and NS–NS sources, respectively. Monte Carlo simulation noise with different models give a nonnegligible uncertainty.

The regular dynamics through the finite‐time Lyapunov exponent distributions in 3D Hamiltonian systems

AbstractWe investigate the application as a technique of the so‐called finite‐time Lyapunov exponent, a scalar value that measures the average, or integrated, separation between trajectories in integrable dynamical systems to quantify the predictability limit of the asymptotic global behavior of old compact star systems (1–10 Gyr), for instance, neutron stars. For this purpose, the Hamiltonian systems with a three‐dimensional self‐gravitating axisymmetric potential are superimposed due to the influence of different birth and kick velocity distributions. We find the behavior of the density distributions P(x) for the finite‐time exponents, which follow a log‐normal distribution (transformed into a Gaussian distribution), with a given mean deviation centered around the global values, at larger Δt. It is shown that simple models of distributions reflect the underlying dynamics. We also analyze the predictability and the deviation vector ω(t), obtained from the distributions, by randomly selecting the initial deviation directions, and consequently the stellar dynamic stability of the periodic orbits, in three‐dimensional (3D) Hamiltonian systems over time. The behavior of the orbits is strongly influenced by the regular nature of the motion.

Neutron Star Mergers: Probing the EoS of Hot, Dense Matter by Gravitational Waves

Gravitational waves, electromagnetic radiation, and the emission of high energy particles probe the phase structure of the equation of state of dense matter produced at the crossroad of the closely related relativistic collisions of heavy ions and of binary neutron stars mergers. 3 + 1 dimensional special- and general relativistic hydrodynamic simulation studies reveal a unique window of opportunity to observe phase transitions in compressed baryon matter by laboratory based experiments and by astrophysical multimessenger observations. The astrophysical consequences of a hadron-quark phase transition in the interior of a compact star will be focused within this article. Especially with a future detection of the post-merger gravitational wave emission emanated from a binary neutron star merger event, it would be possible to explore the phase structure of quantum chromodynamics. The astrophysical observables of a hadron-quark phase transition in a single compact star system and binary hybrid star merger scenario will be summarized within this article. The FAIR facility at GSI Helmholtzzentrum allows one to study the universe in the laboratory, and several astrophysical signatures of the quark-gluon plasma have been found in relativistic collisions of heavy ions and will be explored in future experiments.

Hyperon Effects on the Spin Parameter of Rotating Neutron Stars**Supported by the National Natural Science Foundation of China under Grant Nos 11175108, U1432119, 1146114100, 11205075, 11375076 and 11475104, the Shandong Natural Science Foundation under Grant No ZR2014AQ012, and the Foundation of Shandong University under Grant No 2015WHWLJH01.

Based on the equations of state from the relativistic mean field theory without and with the inclusion of strangeness-bearing hyperons, we study the dimensionless spin parameter j = cJ/(GM2) of uniformly rotating neutron stars. It is shown that the maximum value of the spin parameter jmax of a neutron star rotating at the Keplerian frequency fK is jmax ∼ 0.7 when the star mass M > 0.5M⊙, which is sustained for various versions of equations of state without and with hyperons. The relationship between j and the scaled rotation frequency f/fK is found to be insensitive to the star mass or the adopted equation of state in the models without hyperons. However, the emergence of hyperons in neutron stars will lead to an uncertainty of the spin parameter j, which in turn could generate a complexity in the theoretical study of the quasi-periodic oscillations observed in disk-accreting compact-star systems.

New code for quasiequilibrium initial data of binary neutron stars: Corotating, irrotational, and slowly spinning systems

We present the extension of our cocal---Compact Object CALculator---code to compute general-relativistic initial data for binary compact-star systems. In particular, we construct quasiequilibrium initial data for equal-mass binaries with spins that are either aligned or antialigned with the orbital angular momentum. The Isenberg-Wilson-Mathews formalism is adopted and the constraint equations are solved using the representation formula with a suitable choice of a Green's function. We validate the new code with solutions for equal-mass binaries and explore its capabilities for a wide range of compactnesses, from a white dwarf binary with compactness $\ensuremath{\sim}1{0}^{\ensuremath{-}4}$, up to a highly relativistic neutron-star binary with compactness $\ensuremath{\sim}0.22$. We also present a comparison with corotating and irrotational quasiequilibrium sequences from the spectral code lorene [Taniguchi and Gourgoulhon, Phys. Rev. D 66, 104019 (2002)] and with different compactness, showing that the results from the two codes agree to a precision of the order of 0.05%. Finally, we present equilibria for spinning configurations with a nuclear-physics equation of state in a piecewise polytropic representation.

Constructing effective one-body dynamics with numerical energy flux for intermediate-mass-ratio inspirals

A new scheme for computing dynamical evolutions and gravitational radiations for intermediate-mass-ratio inspirals (IMRIs) based on an effective one-body (EOB) dynamics plus Teukolsky perturbation theory is built in this paper. In the EOB framework, the dynamic essentially affects the resulted gravitational waveform for a binary compact star system. This dynamic includes two parts. One is the conservative part, which comes from effective one-body reduction. The other part is the gravitational backreaction, which contributes to the shrinking process of the inspiral of a binary compact star system. Previous works used an analytical waveform to construct this backreaction term. Since the analytical form is based on post-Newtonian expansion, the consistency of this term is always checked by numerical energy flux. Here, we directly use numerical energy flux by solving the Teukolsky equation via the frequency-domain method to construct this backreaction term. The conservative correction to the leading order terms in mass-ratio is included in the deformed-Kerr metric and the EOB Hamiltonian. We try to use this method to simulate not only quasicircular adiabatic inspiral, but also the nonadiabatic plunge phase. For several different spinning black holes, we demonstrate and compare the resulted dynamical evolutions and gravitational waveforms.

Birth and life of massive black holes

The problems of massive black holes in galactic nuclei of different types are reviewed. The dynamical evolution of compact star systems ends naturally in a gigantic concentrated mass of gas, containing an admixture of surviving stars, that unavoidably collapses into a black hole. The subsequent joint evolution of the remnant star system with a massive black hole at the center leads either to the phenomenon of a bright central source in the nuclei of active galaxies and quasars or to the opposite case of a dead frozen black hole in the nucleus of a normal galaxy.

On the Expected Frequency of WR + Compact Star Systems

According to the classical evolutionary scenario for massive close binariesit is qenerally assumed that two WR phases are encountered, i.e. a WR+OB stage and a WR + compact star stage. The survival probability of a massive binary after an instantaneous supernova (SN) explosion critically depends on the assumed asymmetry of the SN explosion, i.e. on the “kick” velocity Vk (Sutantyo, 1978). The purpose of this paper is to make a statistical study on the expected WR + compact star binaries compared to the WR+OB type binaries based on non-conservative massive binary evolution, as a function of Vk which, as we will see, is very poorly known for these types of stars.