arXiv subjects

Chang-Hwan Lee

Publications and source records attributed to Chang-Hwan Lee.

At least 55 records · Page 3Linked to original sources

arXiv2014

Kaon Condensation in Neutron Stars with Skyrme-Hartree-Fock Models

We investigate nuclear matter equations of state in neutron star with kaon condensation. It is generally known that the existence of kaons in neutron star makes the equation of state soft so that the maximum mass of neutron star is not likely to be greater than 2.0 $M_{\odot}$, the maximum mass constrained by current observations. With existing Skyrme force model parameters, we calculate nuclear equations of state and check the possibility of kaon condensation in the core of neutron stars. The results show that even with the kaon condensation, the nuclear equation of state satisfies both the maximum mass and the allowed ranges of mass and radius.

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arXiv2014

Electromagnetic Radiation in Hot QCD Matter: Rates, Electric Conductivity, Flavor Susceptibility and Diffusion

We discuss the general features of the electromagnetic radiation from a thermal hadronic gas as constrained by chiral symmetry. The medium effects on the electromagnetic spectral functions and the partial restoration of chiral symmetry are quantified in terms of the pion densities. The results are compared with the electromagnetic radiation from a strongly interacting quark-gluon plasma in terms of the leading gluon condensate operators. We use the spectral functions as constrained by the emission rates to estimate the electric conductivity, the light flavor susceptibility and diffusion constant across the transition from the correlated hadronic gas to a strongly interacting quark-gluon plasma.

hep-ph↗

arXiv2014

Parameter Estimation of Gravitational Waves from Nonprecessing BH-NS Inspirals with higher harmonics: Comparing MCMC posteriors to an Effective Fisher Matrix

Using the \texttt{lalinference} Markov-chain Monte Carlo parameter estimation code, we examine two distinct nonprecessing black hole-neutron star (BH-NS) binaries with and without higher-order harmonics. Our simulations suggest that higher harmonics provide a minimal amount of additional information, principally about source geometry. Higher harmonics do provide disproportionately more information than expected from the signal power. Our results compare favorably to the "effective Fisher matrix" approach. Extrapolating using analytic scalings, we expect higher harmonics will provide little new information about nonprecessing BH-NS binaries at the signal amplitudes expected for the first few detections. Any study of subdominant degrees of freedom in gravitational wave astronomy can adopt the tools presented here ($V/V_{\rm prior}$ and $D_{KL}$) to assess whether new physics is accessible (e.g., modifications of gravity; spin-orbit misalignment) and if so precisely what information those new parameters provide. For astrophysicists, we provide a concrete illustration of how well parameters of a BH-NS binary can be measured, relevant to the astrophysical interpretation of coincident EM and GW events (e.g., short GRBs). For our fiducial initial-detector example, the individual masses can be determined to lie between $7.11-11.48 M_\odot$ and $1.77-1.276M_\odot$ at greater than 99% confidence, accounting for unknown BH spin. Assuming comparable control over waveform systematics, future measurements of BH-NS binaries can constrain the BH and perhaps NS mass distributions.

gr-qc↗

arXiv2013

Validity of the Effective Fisher matrix for parameter estimation analysis: Comparing to the analytic Fisher matrix

The effective Fisher matrix method recently introduced by Cho et al. is a semi-analytic approach to the Fisher matrix, in which a local overlap surface is fitted by using a quadratic fitting function. Mathematically, the effective Fisher matrix should be consistent with the analytic one at the infinitesimal fitting scale. In this work, using the frequency-domain waveform (TaylorF2), we give brief comparison results between the effective and analytic Fisher matrices for several non-spinning binaries consisting of binary neutron stars with masses of (1.4, 1.4)M_sun, black hole-neutron star of (1.4, 10)M_sun, and binary black holes of (5, 5) and (10, 10)M_sun for a fixed signal to noise ratio (SNR=20) and show a good consistency between two methods. We also give a comparison result for an aligned-spin black hole-neutron star binary with a black hole spin of χ=1, where we define new mass parameters (Mc, η^-1, χ^7/2) to find good fitting functions to the overlap surface. The effective Fisher matrix can also be computed by using the time-domain waveforms which are generally more accurate than frequency-domain waveform. We show comparison results between the frequency-domain and time-domain waveforms (TaylorT4) for both the non-spinning aligned-spin binaries.

gr-qc↗

arXiv2013

Application of machine learning algorithms to the study of noise artifacts in gravitational-wave data

The sensitivity of searches for astrophysical transients in data from the LIGO is generally limited by the presence of transient, non-Gaussian noise artifacts, which occur at a high-enough rate such that accidental coincidence across multiple detectors is non-negligible. Furthermore, non-Gaussian noise artifacts typically dominate over the background contributed from stationary noise. These "glitches" can easily be confused for transient gravitational-wave signals, and their robust identification and removal will help any search for astrophysical gravitational-waves. We apply Machine Learning Algorithms (MLAs) to the problem, using data from auxiliary channels within the LIGO detectors that monitor degrees of freedom unaffected by astrophysical signals. The number of auxiliary-channel parameters describing these disturbances may also be extremely large; an area where MLAs are particularly well-suited. We demonstrate the feasibility and applicability of three very different MLAs: Artificial Neural Networks, Support Vector Machines, and Random Forests. These classifiers identify and remove a substantial fraction of the glitches present in two very different data sets: four weeks of LIGO's fourth science run and one week of LIGO's sixth science run. We observe that all three algorithms agree on which events are glitches to within 10% for the sixth science run data, and support this by showing that the different optimization criteria used by each classifier generate the same decision surface, based on a likelihood-ratio statistic. Furthermore, we find that all classifiers obtain similar limiting performance, suggesting that most of the useful information currently contained in the auxiliary channel parameters we extract is already being used.

astro-ph.IM↗

arXiv2012

Gravitational waves from BH-NS binaries: Effective Fisher matrices and parameter estimation using higher harmonics

Inspiralling black hole-neutron star (BH-NS) binaries emit a complicated gravitational wave signature, produced by multiple harmonics sourced by their strong local gravitational field and further modulated by the orbital plane's precession. Some features of this complex signal are easily accessible to ground-based interferometers (e.g., the rate of change of frequency); others less so (e.g., the polarization content); and others unavailable (e.g., features of the signal out of band). For this reason, an ambiguity function (a diagnostic of dissimilarity) between two such signals varies on many parameter scales and ranges. In this paper, we present a method for computing an approximate, effective Fisher matrix from variations in the ambiguity function on physically pertinent scales which depend on the relevant signal to noise ratio. As a concrete example, we explore how higher harmonics improve parameter measurement accuracy. As previous studies suggest, for our fiducial BH-NS binaries and for plausible signal amplitudes, we see that higher harmonics at best marginally improve our ability to measure parameters. For non-precessing binaries, these Fisher matrices separate into intrinsic (mass, spin) and extrinsic (geometrical) parameters; higher harmonics principally improve our knowledge about the line of sight. For the precessing binaries, the extra information provided by higher harmonics is distributed across several parameters. We provide concrete estimates for measurement accuracy, using coordinates adapted to the precession cone in the detector's sensitive band.

gr-qc↗

arXiv2011

Holographic equations of state and astrophysical compact objects

We solve the Tolman-Oppenheimer-Volkoff equation using an equation of state (EoS) calculated in holographic QCD. The aim is to use compact astrophysical objects like neutron stars as an indicator to test holographic equations of state. We first try an EoS from a dense D4/D8/\textoverline {D8} model. In this case, however, we could not find a stable compact star, a star satisfying pressure-zero condition with a radius $R$, $p(R)=0$, within a reasonable value of the radius. This means that the EoS from the D4/D8/\textoverline {D8} model may not support any stable compact stars or may support one whose radius is very large. This might be due to a deficit of attractive force from a scalar field or two-pion exchange in the D4/D8/\textoverline {D8} model. Then, we consider D4/D6 type models with different number of quark flavors, $N_f=1,2,3$. Though the mass and radius of a holographic star is larger than those of normal neutron stars, the D4/D6 type EoS renders a stable compact star.

hep-ph↗

arXiv2011

Hyperon and nuclear symmetry energy in the neutron star

In this work, masses and radii of neutron stars are considered to investigate the effect of nuclear symmetry energy to the astrophysical observables. A relativistic mean field model with density-dependent meson-baryon coupling constants is employed in describing the equation of state of dense nuclear matter, and the density dependencies of the symmetry energies are quoted from the recent phenomenological formulae obtained from the heavy ion data at subnuclear saturation densities. Since hyperons can take part in the $β$-equilibrium of the dense matter inside neutron stars, we include hyperons in our estimation and their roles are discussed in combination with that of the nuclear symmetry energy.

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arXiv2008

Supernova 2008D: A Repetition of Supernova 1987A In a Binary

The Supernova 2008D is similar to that of SN 1987A without the H-envelope. Soderberg et al.(2008) reported the serendipitous discovery of the SN2008D at the time of the explosion, accompanied by an X-ray outburst XRF080109. The central remnant, which we believe to be the black-hole (BH) central engine in the Blandford-Znajek mechanism, is estimated, on the basis of the 7% 56Ni production, to have a mass of 1.6-1.8 M_sun. This is not much larger than the upper limit of 1.56 m_sun for the mass of the compact object in SN1987A found by Bethe & Brown (1995); also, on the basis of the 7.5% 56Ni production, they interpreted it as a low-mass BH. Redoing the light curve so as to take into account the absence of convective carbon burning from zero age main sequence 18-24 M_sun and replacing it by carbon shell burning, we see that the remnant in SN2008D must be less massive than in SN 1987A; there of 18 M_sun. Thus, the maximum neutron star mass is 1.5 M_sun. Note that the metallicity of the host galaxy of SN2008D is similar to that of our Galaxy.

astro-ph↗

arXiv2008

Recent Developments on Kaon Condensation and Its Astrophysical Implications

We discuss three different ways to arrive at kaon condensation at n_c = 3 n_0 where n_0 is nuclear matter density: (1) Fluctuating around the n=0 vacuum in chiral perturbation theory, (2) fluctuating around n_VM near the chiral restoration density n_chi where the vector manifestation of hidden local symmetry is reached and (3) fluctuating around the Fermi liquid fixed point at n_0. They all share one common theoretical basis, "hidden local symmetry." We argue that when the critical density n_c < n_chi is reached in a neutron star, the electrons turn into K^- mesons, which go into an S-wave Bose condensate. This reduces the pressure substantially and the neutron star goes into a black hole. Next we develop the argument that the collapse of a neutron star into a black hole takes place for a star of M = 1.5 M_sun. This means that Supernova 1987A had a black hole as result. We also show that two neutron stars in a binary have to be within 4% of each other in mass, for neutron stars sufficiently massive that they escape helium shell burning. For those that are so light that they do have helium shell burning, after a small correction for this they must be within 4% of each other in mass. Observations support the proximity in mass inside of a neutron star binary. The result of strangeness condensation is that there are 5 times more low-mass black-hole, neutron-star binaries than double neutron-star binaries although the former are difficult to observe.

hep-ph↗

arXiv2007

Red-Shift Distribution of Gamma-ray Bursts and Their Progenitors

Gamma ray bursts have been divided into two classes, long-soft gamma ray burst and short-hard gamma ray burst according to the bimodal distribution in duration time. Due to the harder spectrum and the lack of afterglows of short-hard bursts in optical and radio observations, different progenitors for short-hard bursts and long-soft bursts have been suggested. Based on the X-ray afterglow observation and the cumulative red-shift distribution of short-hard bursts, Nakar et al. (2006) found that the progenitors of short-hard bursts are consistent with old populations, such as mergers of binary neutron stars. Recently, the existence of two subclasses in long-soft bursts has been suggested after considering multiple characteristics of gamma-ray bursts, including fluences and the duration time. In this work, we extended the analysis of cumulative red-shift distribution to two possible subclasses in L-GRBs. We found that two possible subclass GRBs show different red-shift distributions, especially for red-shifts z > 1. Our results indicate that the accumulative red-shift distribution can be used as a tool to constrain the progenitor characteristics of possible subclasses in L-GRBs.

astro-ph↗

arXiv2007

Holographic Nuclear Matter in AdS/QCD

We study the physics with finite nuclear density in the framework of AdS/QCD with holographic baryon field included. Based on a mean field type approach, we introduce the nucleon density as a bi-fermion condensate of the lowest mode of the baryon field and calculate the density dependence of the chiral condensate and the nucleon mass. We observe that the chiral condensate as well as the mass of nucleon decrease with increasing nuclear density. We also consider the mass splitting of charged vector mesons in iso-spin asymmetric nuclear matter.

hep-ph↗

arXiv2006

STAR $ρ^0/π^-$ Ratio in Au-Au Peripheral Collisions at RHIC and the Vector Manifestation of Hidden Local Symmetry

The hidden local symmetry effective theory, Wilsonian matched to QCD at a matching scale near the chiral scale $\sim 4πf_π$ as formulated by Harada and Yamawaki, presents a scenario drastically different from the standard scenario of heavy-ion collisions in that as the temperature approaches the critical temperature $T_c$ of chiral restoration from below, the mass of the vector mesons and their widths go to zero in the chiral limit or nearly zero otherwise, proportionally to some power of the quark condensate at what is called "vector manifestation (VM) fixed point". This scenario makes a striking prediction that as the fire-ball expands and cools from $T_c$ down toward the freezeout temperature $T_{freezeout}$ in heavy ion collisions, what we call "hadronic freedom" sets in in which hadrons do not interact or barely interact. We suggest that this scenario is testable at RHIC and as a specific example, we provide an astonishingly simple explanation in terms of the vector manifestation suitably extended to include axial-vector mesons for the anomalously large $ρ^0/π^-$ ratio measured in peripheral collisions by STAR.

nucl-th↗

arXiv2006

Late Hadronization and Matter Formed at RHIC: Vector Manifestation, Brown-Rho Scaling and Hadronic Freedom

Recent developments in our description of RHIC and related heavy-ion phenomena in terms of hidden local symmetry theories are reviewed with a focus on the novel nearly massless states in the vicinity of -- both below and above -- the chiral restoration temperature T_c. We present complementary and intuitive ways to understand both Harada-Yamawaki's vector manifestation structure and Brown-Rho scaling -- which are closely related -- in terms of "melting" of soft glues observed in lattice calculations and join the massless modes that arise in the vector manifestation (in the chiral limit) just below T_c to tightly bound massless states above T_c. This phenomenon may be interpreted in terms of the Beg-Shei theorem. It is suggested that hidden local symmetry theories arise naturally in holographic dual QCD from string theory, and a clear understanding of what really happens near the critical point could come from a deeper understanding of the dual bulk theory. Other matters discussed are the relation between Brown-Rho scaling and Landau Fermi-liquid fixed point parameters at the equilibrium density, its implications for "low-mass dileptons" produced in heavy-ion collisions, the reconstruction of vector mesons in peripheral collisions, the pion velocity in the vicinity of the chiral transition point, kaon condensation viewed from the VM fixed point, nuclear physics with Brown-Rho scaling, and the generic feature of dropping masses at the RGE fixed points in generalized hidden local symmetry theories.

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arXiv2005

Double Neutron Star Binaries: Implications for LIGO

Double neutrons are especially important because they give most accurate informations on the masses of neutron stars. Observations on double neutron stars show that all masses of the neutron stars are below 1.5$\msun$. Furthermore, two neutron stars in a given double pulsar are nearly equal in mass. With hypercritical accretion, we found that the probability of having companion mass $>1.5\msun$ is larger than 90%, while there is no observations on such systems. We believe that those companions with masses higher than $1.5\msun$ went into black holes, which is consistent with our preferred maximum neutron star mass $M_{NS}^{max} \approx 1.5\msun$ due to the kaon condensation. In this work, we point out that the black-hole neutron star binaries are 10 times more dominant than double neutron star binaries. As a result, black-hole, neutron star binaries can increase the LIGO detection rate by a factor 20.

astro-ph↗

arXiv2005

The Problem of Mass: Mesonic Bound States Above T_c

We discuss the problem of mass, noting that meson masses decrease with increasing scale as the dynamically generated condensate of "soft glue" is melted (Brown/Rho scaling). We then extend the Bielefeld LGS color singlet interaction computed for heavy quarks in a model-dependent way by including the Ampere law velocity-velocity interaction. Parameterizing the resulting interaction in terms of effective strength of the potential and including screening, we find that the masses of pi, sigma, rho and A1 excitations, 32 degrees of freedom in all, go to zero (in the chiral limit) as T goes to Tc essentially independently of the input quark (thermal) masses in the range of 1-2 GeV, calculated also in Bielefeld. We discuss other LGS which show q-bar q bound states, which we interpret as our chirally restored mesons, for T > Tc.

hep-ph↗

arXiv2005

The Ideal Liquid Discovered by RHIC, Infrared Slavery Above and Hadronic Freedom Below $T_c$

We construct the nature of the matter found in RHIC when its temperature has dropped down close to, and below, $T_c$. Just above $T_c$ it is composed of extremely strongly bound quark-antiquark pairs forming chirally restored mesons of the quantum numbers of the $π, S, ρ$ and $a_1$ with very small size and zero energy and just below $T_c$, it is composed of mesons of the same quantum numbers with zero mass. We invoke infrared slavery for the former and the vector manifestation (VM) of hidden local symmetry for the latter. As the temperature drops below $T_c$, the strongly bound quark-antiquark pairs are ejected into what is basically a region of "hadronic freedom" in which the interactions are zero. Experimental evidences for this are seen in the STAR data.

nucl-th↗

arXiv2005

Relation of Strangeness Nuggets to Strangeness Condensation and the Maximum Mass of Neutron Stars

Combining the recent experimental indications of density dependence in the pion decay constant $f_π^\star$ and the $ω$ meson mass $m_ω^\star$ and the discovery of $S^0 (3115)$ and other "strange nuggets" with the vector manifestation of chiral symmetry in hidden local symmetry proposed by Harada and Yamawaki, we show that the mechanism responsible for dense strangeness nuggets can be related to that responsible for kaon condensation in neutron-star matter. We suggest that this relation assures kaon condensation at a density $\sim$ three times nuclear matter density which supports the Brown-Bethe scenario for the $M_{\rm NS}^{max} \simeq 1.5\msun$. This low $M_{\rm NS}^{\rm max}\sim 1.5\msun$ has major consequences in astrophysics, especially for the merging rate of compact stellar objects.

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