arXiv subjects

Lap-Ming Lin

Publications and source records attributed to Lap-Ming Lin.

At least 19 recordsLinked to original sources

arXiv2025

Tracing the Trace Anomaly of Dense Matter inside Neutron Stars

The trace anomaly $\Delta$ is an important quantity that measures the broken conformal symmetry in neutron star matter. In this work, we present quasi-universal relations that connect the stellar profile of $\Delta$ to the compactness, moment of inertia, and tidal deformability of neutron stars. We apply the quasi-universal relations to determine the trace anomaly profiles for PSR J0030+0451 and PSR J0740+6620 based on their mass-radius measurements. We also analyze PSR J0737-3039A according to its moment of inertia inferred from Bayesian modeling of nuclear equation of state. A recent multimessenger constraint on the tidal deformability is also studied, resulting in an estimate value of the trace anomaly $\Delta_c = 0.1770^{+0.0365}_{-0.0432}$ at the center of a $1.4M_\odot$ canonical neutron star. It is expected that more precise observations from both electromagnetic and gravitational-wave channels in the future will provide tighter constraints on the behavior of $\Delta$ inside neutron stars.

astro-ph.HE↗

arXiv2025

I-Love-Q Relations of Fermion-Boson Stars

We investigate the properties of fermion-boson stars (FBSs), which can be viewed as neutron stars with a bosonic dark matter (DM) admixture. A challenge in studying the impact of DM on neutron stars is the absence of a universally accepted nuclear-matter equation of state (EOS), making it difficult to distinguish between the effects of DM and various EOS models. To address this issue, we extend the study of the I-Love-Q universal relations of neutron stars to FBSs with a nonrotating bosonic component by solving the Einstein-Klein-Gordon system. We study how DM parameters, such as the boson particle mass and self-interaction strength, would affect the structure of FBSs and explore the parameter space that leads to deviations from the I-Love-Q relations. The properties of FBSs and the level of deviations in general depend sensitively on the DM parameters. For boson particle mass within the range of $\mathcal{O}(10^{-10} \ \mathrm{eV})$, where the Compton wavelength is comparable to the Schwarzschild radius of a $1 M_\odot$ star, the deviation is up to about 5% level if the star contains a few percent of DM admixture. The deviation increases significantly with a higher amount of DM. We also find that the universal relations are still valid to within a 1% deviation level for boson particle mass $m_b \geq 26.8\times10^{-10} \ \mathrm{eV}$. This effectively sets an upper bound on the boson particle mass, beyond which it becomes not feasible to probe the properties of FBSs by investigating the I-Love-Q relation violations.

astro-ph.HE↗

arXiv2024

Revisiting the apparent horizon finding problem with multigrid methods

Apparent horizon plays an important role in numerical relativity as it provides a tool to characterize the existence and properties of black holes on three-dimensional spatial slices in 3+1 numerical spacetimes. Apparent horizon finders based on different techniques have been developed. In this paper, we revisit the apparent horizon finding problem in numerical relativity using multigrid-based algorithms. We formulate the nonlinear elliptic apparent horizon equation as a linear Poisson-type equation with a nonlinear source, and solve it using a multigrid algorithm with Gauss-Seidel line relaxation. A fourth order compact finite difference scheme in spherical coordinates is derived and employed to reduce the complexity of the line relaxation operator to a tri-diagonal matrix inversion. The multigrid-based apparent horizon finder developed in this work is capable of locating apparent horizons in generic spatial hypersurfaces without any symmetries. The finder is tested with both analytic data, such as Brill-Lindquist multiple black hole data, and numerical data, including off-centered Kerr-Schild data and dynamical inspiraling binary black hole data. The obtained results are compared with those generated by the current fastest finder AHFinderDirect (Thornburg, Class. Quantum Grav. 21, 743, 2003), which is the default finder in the open source code Einstein Toolkit. Our finder performs comparatively in terms of accuracy, and starts to outperform AHFinderDirect at high angular resolutions (\sim 1^\circ) in terms of speed. Our finder is also more flexible to initial guess, as opposed to the Newton's method used in AHFinderDirect. This suggests that the multigrid algorithm provides an alternative option for studying apparent horizons, especially when high resolutions are needed.

gr-qc↗

arXiv2024

On the Nucleosynthesis in Accretion-Induced Collapse of White Dwarfs

It has long been hypothesized that accretion-induced collapse (AIC) of white dwarfs contribute to heavy chemical elements production in the universe. We present one-dimensional neutrino-radiative hydrodynamic simulations of AIC followed by post-processing nucleosynthesis calculations of the ejecta. A proto-neutron star is formed after the AIC, and a neutrino burst with peak luminosity $\sim10^{53}$ erg s$^{-1}$, comparable to that of a core-collapse supernova (CCSN), is emitted. The ejecta mass of AIC could be up to $\sim10^{-2}$ M$_\odot$, and the first neutron-capture peak elements (Sr, Y, and Zr) could be abundantly synthesized, with an overproduction of $\sim10^{6}$ relative to the solar abundances. The yield of $^{56}\text{Ni}$ could be up to at most $\sim10^{-3}$ M$_\odot$, suggesting that the electromagnetic light curve associated with AIC is at least $2$ orders dimmer than those associated with Type Ia supernovae (Type Ia SN). The inferred upper bound of AIC event rate, from nucleosynthesis calculations, is at most $\sim10\,\%$ relative to those of CCSNe and Type Ia SNe.

astro-ph.HE↗

arXiv2023

Torsional oscillations of magnetized neutron stars: Impacts of Landau-Rabi quantization of electron motion

Torsional oscillations of magnetized neutron stars have been well studied since they may be relevant to the physical interpretation of some of the observed quasiperiodic oscillations in the magnetar giant flares. In the crustal region of a magnetar, the strong magnetic field can alter the equation of state and composition due to the Landau-Rabi quantization of electron motion. In this paper, we study this effect on the crust-confined, torsional oscillation modes of neutron stars with mixed poloidal-toroidal magnetic fields in general relativity under the Cowling approximation. Furthermore, the inner and outer crusts are treated consistently based on the nuclear-energy density functional theory. Depending on the magnetic-field configurations, we find that the Landau-Rabi quantization of electrons can change the frequencies of the fundamental torsional oscillation mode of $1.4 M_\odot$ neutron star models with a normal fluid core by about 10% when the magnetic field strength at the pole reaches the order of $10^{16}$ G. The shift can even approach 20% at a field strength of $10^{15}$ G for neutron stars with a simple model of superconducting core where the magnetic field is assumed to be expelled completely.

astro-ph.HE↗

arXiv2023

Fully general relativistic simulations of rapidly rotating quark stars: Oscillation modes and universal relations

(Abridged) Numerical simulation of strange quark stars (QSs) is challenging due to the strong density discontinuity at the stellar surface. In this paper, we report successful simulations of rapidly rotating QSs and study their oscillation modes in full general relativity. Building on top of the numerical relativity code \texttt{Einstein Toolkit}, we implement a positivity-preserving Riemann solver and a dust-like atmosphere to handle the density discontinuity at the surface. We demonstrate the robustness of our numerical method by performing stable evolutions of rotating QSs close to the Keplerian limit and extracting their oscillation modes. We focus on the quadrupolar $l=|m|=2$ $f$-mode and study whether they can still satisfy the universal relations recently proposed for rotating neutron stars (NSs). We find that two of the three proposed relations can still be satisfied by rotating QSs. For the remaining broken relation, we propose a new relation to unify the NS and QS data by invoking the dimensionless spin parameter $j$. The onsets of secular instabilities for rotating QSs are also studied by analyzing the $f$-mode frequencies. Same as the result found previously for NSs, we find that QSs become unstable to the Chandrasekhar-Friedman-Schutz instability when the angular velocity of the star $\Omega \approx 3.4 \sigma_0$ for sequences of constant central energy density, where $\sigma_0$ is the mode frequency of the corresponding nonrotating configurations. For the viscosity-driven instability, we find that QSs become unstable when $j\approx 0.881$ for both sequences of constant central energy density and constant baryon mass. Such a high value of $j$ cannot be achieved by realistic rotating NSs before reaching the Keplerian limit.

gr-qc↗

arXiv2022

Oscillations and tidal deformations of crystallized white dwarfs

Long predicted more than fifty years ago, strong evidence for the existence of crystalline cores inside white dwarfs has recently been obtained by the Gaia space telescope. It is thus important to investigate how a crystalline core may affect the properties and dynamics of white dwarfs. In this paper, we first study the dependence of the frequencies of the fundamental (f), interfacial (i), and shear (s) oscillation modes on the size of the crystalline core. We find that the frequencies of the i- and s-modes depend sensitively on the size of the core, while the frequency of the f-mode is affected only slightly by at most a few percent for our chosen white dwarf models. We next consider the tidal deformability of crystallized white dwarfs and find that the effect of crystallization becomes significant only when the radius of the core is larger than about 70% of the stellar radius. The tidal deformability can change by a few to about 10 percent when a white dwarf becomes fully crystallized. We also show that there exist approximate equation-of-state insensitive relations connecting the mass, moment of inertia, tidal deformability, and f-mode frequency for pure fluid white dwarfs. Depending on the stellar mass and composition, however, these relations can be affected by a few percent when the white dwarf is crystallized. These changes could leave an imprint on the gravitational waves emitted from the late inspiral or merger of white dwarf binaries, which may be detectable by future space-borne gravitational wave detectors.

astro-ph.SR↗

arXiv2022

R-process Nucleosynthesis of Subminimal Neutron Star Explosions

We show that a minimum-mass neutron star undergoes delayed explosion after mass removal from its surface. We couple the Newtonian hydrodynamics to a nuclear reaction network of $\sim4500$ isotopes to study the nucleosynthesis and neutrino emission during the explosion. An electron antineutrino burst with a peak luminosity of $\sim3\times10^{50}$ erg s$^{-1}$ is emitted while the ejecta is heated to $\sim10^{9}$ K. A robust $r$-process nucleosynthesis is realized in the ejecta. Lanthanides and heavy elements near the second and third $r$-process peaks are synthesized as end products of nucleosynthesis, suggesting that subminimal neutron star explosions could be an important source of solar chemical elements.

astro-ph.HE↗

arXiv2022

Tidal Deformability of Dark Matter Admixed Neutron Stars

The tidal properties of a neutron star are measurable in the gravitational waves emitted from inspiraling binary neutron stars, and they have been used to constrain the neutron star equation of state. In the same spirit, we study the dimensionless tidal deformability of dark matter admixed neutron stars. The tidal Love number is computed in a two-fluid framework. The dimensionless tidal Love number and dimensionless tidal deformability are computed for dark matter admixed stars with the dark matter modelled as ideal Fermi gas or self-interactive bosons. The dimensionless tidal deformability shows a sharp change from being similar to that of a pure normal matter star to that of a pure dark matter star, within a narrow range of intermediate dark matter mass fraction. Based on this result, we illustrate an approach to study the dark matter parameters through the tidal properties of massive compact stars, making use of the self-similarity of the dimensionless tidal deformability-mass relations when the dark matter mass fraction is high.

astro-ph.HE↗

arXiv2021

Neutron star oscillations in pseudo-Newtonian gravity

We investigate the oscillations of neutron stars using a purely Newtonian approach and three other pseudo-Newtonian formulations. Our work is motivated by the fact that pseudo-Newtonian formulations are commonly used in core-collapse supernova (CCSN) simulations. We derive and solve numerically the radial and nonradial perturbation equations for neutron star oscillations using different combinations of modified Newtonian hydrodynamics equations and gravitational potentials. We pay special attention to the formulation proposed recently by Zha et al. [Phys. Rev. Lett. 125, 051102 (2020)] that implements the standard Case A effective potential in CCSN simulations with an additional lapse-function correction to the hydrodynamics equations. We find that this "Case A+lapse" formulation can typically approximate the frequency of the fundamental radial mode of a $1.4 M_\odot$ neutron star computed in general relativity to about a few tens of percent for our chosen EOS models. For the nonradial quadrupolar $f$ mode, which is expected to contribute strongly to the gravitational waves emitted from a protoneutron star, the Case A+lapse formulation performs much better and can approximate the $f$ mode frequency to within about a few percent even for the maximum-mass configuration in general relativity.

astro-ph.HE↗

arXiv2021

Can the GW190814 secondary component be a bosonic dark matter admixed compact star?

We investigate whether the recently observed 2.6 $M_\odot$ compact object in the gravitational-wave event GW190814 can be a bosonic dark matter admixed compact star. By considering the three constraints in mass, radius and stability of such an object, we find that if the dark matter is made of QCD axions, their particle mass $m$ is constrained to a range that has already been ruled out by the independent constraint imposed by the stellar-mass black hole superradiance process. The 2.6 $M_\odot$ object can still be a neutron star admixed with at least 2.0 $M_\odot$ of dark matter made of axion-like particles (or even a pure axion-like particle star) if $2 \times 10^{-11}$ eV $\leq m \leq 2.4 \times 10^{-11}$ eV ($2.9 \times 10^{-11}$ eV $\leq m \leq 3.2 \times 10^{-11}$ eV) and with decay constant $f \geq 8 \times 10^{17}$ GeV.

astro-ph.HE↗

arXiv2021

The I-Love-Q Relations for Superfluid Neutron Stars

The I-Love-Q relations are approximate equation-of-state independent relations that connect the moment of inertia, the spin-induced quadrupole moment, and the tidal deformability of neutron stars. In this paper, we study the I-Love-Q relations for superfluid neutron stars for a general relativistic two-fluid model: one fluid being the neutron superfluid and the other a conglomerate of all charged components. We study to what extent the two-fluid dynamics might affect the robustness of the I-Love-Q relations by using a simple two-component polytropic model and a relativistic mean field model with entrainment for the equation-of-state. Our results depend crucially on the spin ratio $\Omega_{\rm n}/\Omega_{\rm p}$ between the angular velocities of the neutron superfluid and the normal component. We find that the I-Love-Q relations can still be satisfied to high accuracy for superfluid neutron stars as long as the two fluids are nearly co-rotating $\Omega_{\rm n}/\Omega_{\rm p} \approx 1$. However, the deviations from the I-Love-Q relations increase as the spin ratio deviates from unity. In particular, the deviation of the Q-Love relation can be as large as $O(10\%)$ if $\Omega_{\rm n}/\Omega_{\rm p}$ differ from unity by a few tens of percent. As $\Omega_{\rm n}/\Omega_{\rm p} \approx 1$ is expected for realistic neutron stars, our results suggest that the two-fluid dynamics should not affect the accuracy of any gravitational waveform models for neutron star binaries that employ the relation to connect the spin-induced quadrupole moment and the tidal deformability.

astro-ph.HE↗

arXiv2020

Gravitational-wave asteroseismology with f-modes from neutron star binaries at the merger phase

Gravitational-wave signals from coalescing binary neutron stars can yield important information about the properties of nuclear-matter equation of state from the early part of the signal through tidal effects to the properties and oscillation frequencies of the merger product. We investigate a direct link between the properties of isolated neutron stars and their merger, by comparing the frequency of the fundamental oscillation mode ($f$-mode) of neutron stars with the gravitational-wave frequency associated with the merger of two neutron stars. We calculate the quadrupolar ($l=2$) $f$-mode oscillation ($f_{2f}$) of non-rotating and rotating neutron stars using a nonlinear hydrodynamics code in the conformally-flat approximation and obtain the gravitational-wave frequency associated with the peak amplitude ($f_{\rm max}$) of binary-neutron stars from a set of publicly available simulations. We find that $f_{\rm max}$ and $f_{2f}$ differ by about 1\%, on average, across forty-five equal-mass systems with different total mass and equations of state. Assuming that the gravitational-wave frequency is still approximately equal to twice the orbital frequency $\Omega$ near the merger, the result indicates that the condition for tidal resonance $|m|\Omega = f_{2f}$ is satisfied to high accuracy near the merger, where $m=2$ is the azimuthal quantum number. Moreover, the well established universal relation between $f_{\rm max}$ and the tidal deformability of equal-mass binary systems can now be explained by a similar relation between $f_{2f}$ and the tidal deformability of isolated neutron stars, which has been demonstrated to be associated with the nearly incompressible properties of neutron stars. Our findings suggest that it is possible to relate the gravitational-wave signal at the merger of a binary neutron star system directly to the fundamental oscillation modes and the mass ratio.

astro-ph.HE↗

arXiv2020

Gravitational-wave Signature of a First-order Quantum Chromodynamics Phase Transition in Core-Collapse Supernovae

A first-order quantum chromodynamics (QCD) phase transition (PT) may take place in the protocompact star (PCS) produced by a core-collapse supernova (CCSN). In this work, we study the consequences of such a PT in a non-rotating CCSN with axisymmetric hydrodynamic simulations. We find that the PT leads to the collapse of the PCS and results in a loud burst of gravitational waves (GWs). The amplitude of this GW burst is $\sim30$ times larger than the post-bounce GW signal normally found for non-rotating CCSN. It shows a broad peak at high frequencies ($\sim2500-4000$ Hz) in the spectrum, has a duration of $\lesssim5 {\rm ms}$, and carries $\sim3$ orders of magnitude more energy than the other episodes. Also, the peak frequency of the PCS oscillation increases dramatically after the PT-induced collapse. In addition to a second neutrino burst, the GW signal, if detected by the ground-based GW detectors, is decisive evidence of the first-order QCD PT inside CCSNe and provides key information about the structure and dynamics of the PCS.

astro-ph.HE↗

arXiv2020

Gmunu: Toward multigrid based Einstein field equations solver for general-relativistic hydrodynamics simulations

We present a new open-source axisymmetric general relativistic hydrodynamics code Gmunu (General-relativistic multigrid numerical solver) which uses a multigrid method to solve the elliptic metric equations in the conformally flat condition (CFC) approximation on a spherical grid. Most of the existing relativistic hydrodynamics codes are based on formulations which rely on a free-evolution approach of numerical relativity, where the metric variables are determined by hyperbolic equations without enforcing the constraint equations in the evolution. On the other hand, although a fully constrained-evolution formulation is theoretical more appealing and should lead to more stable and accurate simulations, such an approach is not widely used because solving the elliptic-type constraint equations during the evolution is in general more computationally expensive than hyperbolic free-evolution schemes. Multigrid methods solve differential equations with a hierarchy of discretizations and its computational cost is generally lower than other methods such as direct methods, relaxation methods, successive over-relaxation. With multigrid acceleration, one can solve the metric equations on a comparable time scale as solving the hydrodynamics equations. This would potentially make a fully constrained-evolution formulation more affordable in numerical relativity simulations. As a first step to assess the performance and robustness of multigrid methods in relativistic simulations, we develop a hydrodynamics code that makes use of standard finite-volume methods coupled with a multigrid metric solver to solve the Einstein equations in the CFC approximation. In this paper, we present the methodology and implementation of our code Gmunu and its properties and performance in some benchmarking relativistic hydrodynamics problems.

gr-qc↗

arXiv2019

Accretion-Induced Collapse of Dark Matter Admixed White Dwarfs -- II: Rotation and Gravitational-wave Signals

We present axisymmetric hydrodynamical simulations of accretion-induced collapse (AIC) of dark matter (DM) admixed rotating white dwarfs (WD) and their burst gravitational-wave (GW) signals. For initial WD models with the same central baryon density, the admixed DM is found to delay the plunge and bounce phases of AIC, and decrease the central density and mass of the proto-neutron star (PNS) produced. The bounce time, central density and PNS mass generally depend on two parameters, the admixed DM mass $M_\mathrm{DM}$ and the ratio between the rotational kinetic and gravitational energies of the inner core at bounce $\beta_\mathrm{ic,b}$. The emitted GWs have generic waveform shapes and the variation of their amplitudes $h_+$ show a degeneracy on $\beta_\mathrm{ic,b}$ and $M_\mathrm{DM}$. We found that the ratios between the GW amplitude peaks around bounce allow breaking the degeneracy and extraction of both $\beta_\mathrm{ic,b}$ and $M_\mathrm{DM}$. Even within the uncertainties of nuclear matter equation of state, a DM core can be inferred if its mass is greater than 0.03 $M_{\odot}$. We also discuss possible DM effects on the GW signals emitted by PNS g-mode oscillations. GW may boost the possibility for the detection of AIC, as well as open a new window in the indirect detection of DM.

astro-ph.HE↗

arXiv2019

Accretion-Induced Collapse of Dark Matter Admixed White Dwarfs -- I: Formation of Low-mass Neutron Stars

Recently observed pulsars with masses $\sim 1.1 ~M_{\odot}$ challenge the conventional neutron star (NS) formation path by core-collapse supernova (CCSN). Using spherically symmetric hydrodynamics simulations, we follow the collapse of a massive white dwarf (WD) core triggered by electron capture, until the formation of a proto-NS (PNS). For initial WD models with the same central density, we study the effects of a static, compact dark matter (DM) admixed core on the collapse and bounce dynamics and mass of the PNS, with DM mass $\sim 0.01 ~M_{\odot}$. We show that increasing the admixed DM mass generally leads to slower collapse and smaller PNS mass, down to about 1.0 $M_{\odot}$. Our results suggest that the accretion-induced collapse of dark matter admixed white dwarfs can produce low-mass neutron stars, such as the observed low-mass pulsar J0453+1559, which cannot be obtained by conventional NS formation path by CCSN.

astro-ph.HE↗

arXiv2018

Universal relations for innermost stable circular orbits around rapidly rotating neutron stars

We study the innermost stable circular orbit (ISCO) of a test particle around rapidly rotating neutron stars. Based on 12 different nuclear-matter equations of state (EOS), we find numerically two approximately EOS-insensitive universal relations that connect the radius and orbital frequency of the ISCO to the spin frequency $f$ and mass $M$ of rotating neutron stars. The relations are EOS-insensitive to about the 2% level for a large range of $Mf$. We also find that the universal relation for the ISCO radius agrees with the corresponding relation for the Kerr black hole to within 6% up to $Mf=5000 M_\odot {\rm Hz}$. Our relations can be applied to accreting neutron stars in low-mass X-ray binaries. Using the spin frequency $f=414$ Hz and the highest kilohertz quasi-periodic oscillations (kHz QPOs) at 1220 Hz observed in the system 4U 0614+09, we determine the mass of the neutron star to be $2.0 M_\odot$. Our conclusion only makes a minimal assumption that the highest kHz QPO frequency is the ISCO frequency, bypassing the assumption of slow rotation and the uncertainty related to the dimensionless spin parameter, which are commonly required in the literature.

astro-ph.HE↗