arXiv subjects

Lap-Ming Lin

Publications and source records attributed to Lap-Ming Lin.

28 records · Page 2Linked to original sources

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↗

arXiv2014

Torsional oscillations of crystalline color-superconducting hybrid stars: Possible sources for Advanced LIGO?

Deconfined quark matter may exist in a crystalline color-superconducting phase in the interiors of compact stars. In this paper, we study the torsional oscillations of compact stars featuring a crystalline color-superconducting quark-matter core in general relativity. Depending on the size of the crystalline core and the value of the gap parameter $Δ$, we find that the frequencies of the torsional oscillation modes can range from a few hundred hertz to a few kilohertz for our canonical $1.4 M_\odot$ compact star models. We have also studied the prospect for detecting the gravitational-wave signals emitted from these modes in a pulsar glitch event. Assuming that at least $10\%$ of the energy released in a Vela glitch can be channeled to the oscillation modes, we find that the Einstein Telescope should be able to detect these signals in quite general situations. Furthermore, if the size of the crystalline core is comparable to the stellar radius and the gap parameter is relatively small at $Δ\sim 5$ MeV, the signal-to-noise ratio for Advanced LIGO could reach $\sim$10 for a Vela glitch. Our optimistic results suggest that we might already be able to probe the nature of crystalline color-superconducting quark matter with the second-generation gravitational-wave detectors when they come online in the next few years.

astro-ph.HE↗

arXiv2011

The spin parameter of uniformly rotating compact stars

We study the dimensionless spin parameter $j (= c J/ (G M^2))$ of uniformly rotating neutron stars and quark stars in general relativity. We show numerically that the maximum value of the spin parameter of a neutron star rotating at the Keplerian frequency is $j_{\rm max} \sim 0.7$ for a wide class of realistic equations of state. This upper bound is insensitive to the mass of the neutron star if the mass of the star is larger than about $1 M_\odot$. On the other hand, the spin parameter of a quark star modeled by the MIT bag model can be larger than unity and does not have a universal upper bound. Its value also depends strongly on the bag constant and the mass of the star. Astrophysical implications of our finding will be discussed.

astro-ph.HE↗

arXiv2007

Constraining crystalline color superconducting quark matter with gravitational-wave data

We estimate the maximum equatorial ellipticity sustainable by compact stars composed of crystalline color-superconducting quark matter. For the theoretically allowed range of the gap parameter $Δ$, the maximum ellipticity could be as large as $10^{-2}$, which is about 4 orders of magnitude larger than the tightest upper limit obtained by the recent science runs of the LIGO and GEO600 gravitational wave detectors based on the data from 78 radio pulsars. We point out that the current gravitational-wave strain upper limit already has some implications for the gap parameter. In particular, the upper limit for the Crab pulsar implies that $Δ$ is less than O(20) MeV for a range of quark chemical potential accessible in compact stars, assuming that the pulsar has a mass $1.4 M_{\odot}$, radius 10 km, breaking strain $10^{-3}$, and that it has the maximum quadrupole deformation it can sustain without fracturing.

astro-ph↗

arXiv2007

Oscillations of General Relativistic Multi-fluid/Multi-layer Compact Stars

We develop the formalism for determining the quasinormal modes of general relativistic multi-fluid compact stars in such a way that the impact of superfluid gap data can be assessed. Our results represent the first attempt to study true multi-layer dynamics, an important step towards considering realistic superfluid/superconducting compact stars. We combine a relativistic model for entrainment with model equations of state that explicity incorporate the symmetry energy. Our analysis emphasises the many different parameters that are required for this kind of modelling, and the fact that standard tabulated equations of state are grossly incomplete in this respect. To make progress, future equations of state need to provide the energy density as a function of the various nucleon number densities, the temperature (i.e. entropy), and the entrainment among the various components.

gr-qc↗

arXiv2007

A new spectral apparent horizon finder for 3D numerical relativity

We present a new spectral-method-based algorithm for finding apparent horizons in three-dimensional space-like hypersurfaces without symmetries. While there are already a wide variety of algorithms for finding apparent horizons, our new algorithm does not suffer from the same weakness as previous spectral apparent horizon finders: namely the monopolar coefficient ($\ell=0$ in terms of the spherical harmonics decomposition) needed to be determined by a root-finding procedure. Hence, this leads to a much faster and more robust spectral apparent horizon finder. The finder is tested with the Kerr-Schild and Brill-Lindquist data. Our finder is accurate and is as efficient as the currently fastest methods developed recently by Schnetter (2003 Class. Quantum Grav. {\bf 20}, 4719) and Thornburg (2004 Class. Quantum Grav. {\bf 21}, 743). At typical resolutions it takes only 0.5 second to find the apparent horizon of a Kerr-Schild black hole with $a=0.9M$ to the accuracy $\sim 10^{-5}$ for the fractional error in the horizon's location on a 2 GHz processor.

gr-qc↗

arXiv2006

Rotating star initial data for a constrained scheme in numerical relativity

A new numerical code for computing stationary axisymmetric rapidly rotating stars in general relativity is presented. The formulation is based on a fully constrained-evolution scheme for 3+1 numerical relativity using the Dirac gauge and maximal slicing. We use both the polytropic and MIT bag model equations of state to demonstrate that the code can construct rapidly rotating neutron star and strange star models. We compare numerical models obtained by our code and a well-established code, which uses a different gauge condition, and show that the two codes agree to high accuracy.

gr-qc↗

arXiv2006

Nonlinear r-modes in neutron stars: A hydrodynamical limitation on r-mode amplitudes

Previously we found that large amplitude $r$-modes could decay catastrophically due to nonlinear hydrodynamic effects. In this paper we found the particular coupling mechanism responsible for this catastrophic decay, and identified the fluid modes involved. We find that for a neutron star described by a polytropic equation of state with polytropic index $Γ=2$, the coupling strength of the particular three-mode interaction causing the decay is strong enough that the usual picture of the $r$-mode instability with a flow pattern dominated by that of an $r$-mode can only be valid for the dimensionless $r$-mode amplitude less than $O(10^{-2})$.

gr-qc↗

arXiv2003

Gravitational Waves from Phase-Transition Induced Collapses of Neutron Stars

We study the gravitational wave signals emitted from phase-transition induced collapses of rapidly rotating neutron stars to strange stars by performing 3D numerical simulations. Our preliminary results suggest that the complete conversion of neutron stars to strange stars would occur within a fraction of millisecond. We also find that the gravitational waves generated from the collapse process may be detectable by the advanced LIGO for reasonable source distance. In addition, the study such gravitational wave signals would put useful constraint on the parameters of QCD.

astro-ph↗

arXiv2003

Nonlinear r-Modes in Neutron Stars: Instability of an unstable mode

We study the dynamical evolution of a large amplitude r-mode by numerical simulations. R-modes in neutron stars are unstable growing modes, driven by gravitational radiation reaction. In these simulations, r-modes of amplitude unity or above are destroyed by a catastrophic decay: A large amplitude r-mode gradually leaks energy into other fluid modes, which in turn act nonlinearly with the r-mode, leading to the onset of the rapid decay. As a result the r-mode suddenly breaks down into a differentially rotating configuration. The catastrophic decay does not appear to be related to shock waves at the star's surface. The limit it imposes on the r-mode amplitude is significantly smaller than that suggested by previous fully nonlinear numerical simulations.

gr-qc↗