arXiv subjects

Hyun Kyu Lee

Publications and source records attributed to Hyun Kyu Lee.

At least 73 records · Page 4Linked to original sources

arXiv2004

Representation Mixing and Exotic Baryons in the Skyrme Model

We study the effect of representation mixing in the SU(3) Skyme model by diagonalizing exactly the representation-dependent part. It is observed that even without the next-to-leading order symmetry breaking terms the low-lying baryon masses as well as the recently discovered $Θ+$ and $\Cascade_{\bar{10}}$ can be fairly well reproduced within 3% accuracy. It is also demonstrated that the mixing effect is not negligible in decay processes of $\{\bar{10}\}$. In particular the effect of mixing with $\{27\}$ is found to be quite large. These results are compatible with the second-order perturbation scheme. The decay widths are found to be sensitive to the mass values. The decay widths of $\{\bar{10}\}$ are estimated to be smaller than those of $\{10\}$ by an order of magnitude due to the destuctive interference between operators although the kinematic factors are comparable.

hep-ph↗

arXiv2004

Line-broadening in gravitational radiation from gamma-ray burst-supernovae

Core-collapse in massive stars is believed to produce a rapidly spinning black hole surrounded by a compact disk or torus. This forms an energetic MeV-nucleus inside a remnant He-core, powered by black hole-spin energy. The output produces a GRB-supernova, while most of the energy released is in gravitational radiation. The intrinsic gravitational-wave spectrum is determined by multipole mass-moments in the torus. Quadrupole gravitational radiation is radiated at about twice the Keplerian frequency of the torus, which is non-axisymmetric when sufficiently slender, representing a ``{\em black hole-blob}" binary system or a "{\em blob-blob}" binary bound to the central black hole. We here discuss line-broadening in the observed spectrum due to Lense-Thirring precession, which modulates the orientation of the torus to the line-of-sight. This spectral feature is long-lived, due to weak damping of precessional motion. These events are believed to occur perhaps once per year within a distance of 100Mpc, which provides a candidate source for Advanced LIGO.

astro-ph↗

arXiv2004

Two Dimensional Poynting Flux Dominated Flow onto a Schwarzschild Black Hole

We discuss the dynamics of the accretion flow onto a black hole driven by Poynting flux in a simplified model of a two-dimensional accretion disk. In this simplified model, the condition of the stationary accretion flow is found to impose a nontrivial constraint on the magnetic field configuration. The effect of the magnetic field on the accretion flow is discussed in detail using the paraboloidal and hyperboloidal type configuration for the poloidal structure suggested by Blandford in 1976. It is demonstrated explicitly that the angular velocity of the disk, $Ω_D$, deviates from the Keplerian angular velocity. The angular velocity of the rigidly-rotating magnetic surface, $Ω_F$, does not have to be the same as the angular velocity of the disk for the paraboloidal type configuration. But for the hyperboloidal type configuration, it is found that we can set $Ω_F = Ω_D$, which corresponds to an accretion disk of perfect conductor. We discuss the numerical solutions of the stream equation for stationary accretion flow in the Schwarzschild background using a paraboloidal type configuration. The dynamics of the accretion disk is found to depend strongly on the ratio of the accretion rate to the magnetic field strength.

astro-ph↗

arXiv2003

Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO

Gamma-ray bursts are believed to originate in core-collapse of massive stars. This produces an active nucleus containing a rapidly rotating Kerr black hole surrounded by a uniformly magnetized torus represented by two counter-oriented current rings. We quantify black hole spin-interactions with the torus and charged particles along open magnetic flux-tubes subtended by the event horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii) aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al. 2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating LIGO/Virgo detectors enables searches for nearby events and their spectral closure density 6e-9 around 250Hz in the stochastic background radiation in gravitational waves. At current sensitivity, LIGO-Hanford may place an upper bound around 150MSolar in GRB030329. Detection of Egw thus provides a method for identifying Kerr black holes by calorimetry.

gr-qc↗

arXiv2003

Charges and Magnetic Fields of the Slowly-rotating Neutron Stars

In association with the Goldreich-Julian's magnetic braking mechanism to explain the luminosity of radio pulsars as a result of their spin-down, it is of some interest to explore how much of magnetic flux can actually penetrate the surface of the rotating neutron stars at least in idealized situations. In order to address this issue, one needs to figure out the amount of charge on and the structure of magnetic field around a rotating neutron star. In the present work, based on the solution-generating method given by Wald, the magnetic fields around both the uncharged and (slightly) charged neutron star have been obtained. Particularly for the charged neutron star, it has been demonstrated following again the argument by Wald that the neutron star will gradually accrete the charge until it reaches the equilibrium value $\tilde{Q}=2B_{0}J$. Then next, the magnetic flux through one half of the surface of the rotating neutron star has been computed as well. With the nonvanishing accretion charge having value in the range $0 < \tilde{Q} \leq 2B_{0}J$, the total magnetic flux through the neutron star has been shown to be greater than that without the accretion charge.

astro-ph↗

arXiv2003

Magnetic Alignment Process : A New Mechanism for Extracting Energy from Rotating Black Holes

As a complementary or companion process to the Blandford-Znajek mechanism for the rotational energy extraction from a Kerr black hole to serve as a viable model for the central engine of quasar, AGN and even GRB, the magnetic alignment process is proposed. In contrast to the environment assumed in the Blandford-Znajek mechanism in which the rotating hole's spin axis and the asymptotic direction of the magnetic field are aligned, this new process operates when they are generally misaligned which obviously is the more natural situation likely to happen. The time scales over which these two processes take place and the total radiated powers released at the horizon by them have been estimated and found to be nearly comparable. This may imply that in a random, non-axisymmetric configuration, the magnetic alignment process would operate first during which some of the hole's rotational energy is released and upon completion of the alignment, the Blandford-Znajek mechanism may follow and the rest of the hole's rotational energy, if any left, would be further released.

astro-ph↗

arXiv2002

Thermodynamic constraint on the primordial black hole formation in the radiation dominated epoch

It has been suggested that the overdense region as a result of inhomogeneities in the early Universe would have undergone a collapse into the primordial black holes(PBH). In this work, we discuss a possible constraint on the PBH formation in the radiation dominated epoch by imposing the generalized second law of thermodynamics in the context of spherically collapsing scenario. It is found that both the critical temperature $T_c$ over which the formation of PBH is not possible and the lower bound on the mass of PBH depend on the number of degrees of freedom at the time of PBH formation. In the standard model, one can show that the lower bound on the mass of PBH known in the literature, of order Planck mass, is consistent with the thermodynamic constraint constructed in this work. We also pointed out the possibility that the critical temperature(lower bound on PBH mass) can be lowered(increased) provided the number of relativistic degrees of freedom of the Universe is increasing substantially beyond the standard model.

hep-ph↗

arXiv2002

Radial equation for an accretion flow driven by Poynting flux

Using a toy model of a two dimensional accretion disk, we discuss further the radial equation of the accretion flow dominated by the Poynting flux. Assuming the force-free condition is valid around the accretion disk, the relation between the fluid angular velocity of the disk, $Ω_D$, and the angular velocity of the magnetic field lines, $Ω_F$, is also discussed.

astro-ph↗

arXiv2002

Evolution of a black hole at the center of GRB

Using a simplified model of a black hole-accretion disk system which is dominated by Poynting flux, the evolution of the central black hole which is supposed to be powering GRB is discussed. It is demonstrated explicitly that there is a lower limit on the angular momentum parameter for a given GRB energy. It is found that the most energetic GRBs can only accommodate relatively rapid-rotating black holes at the center. For a set of GRBs for which the isotropic energies and T_90s are known, the effect of the disk mass and the magnetic field on the horizon are discussed quantitatively. It is found that the magnetic field has little influence on the energy but affects the GRB duration as expected. The role of the disk mass is found to be significant in determining both the energy and the duration.

astro-ph↗

arXiv2001

Znajek-Damour Horizon Boundary Conditions with Born-Infeld Electrodynamics

In this work, the interaction of electromagnetic fields with a rotating (Kerr) black hole is explored in the context of Born-Infeld (BI) theory of electromagnetism instead of standard Maxwell theory and particularly BI theory versions of the four horizon boundary conditions of Znajek and Damour are derived. Naturally, an issue to be addressed is then whether they would change from the ones given in Maxwell theory context and if they would, how. Interestingly enough, as long as one employs the same local null tetrad frame as the one adopted in the works by Damour and by Znajek to read out physical values of electromagnetic fields and fictitious surface charge and currents on the horizon, it turns out that one ends up with exactly the same four horizon boundary conditions despite the shift of the electrodynamics theory from a linear Maxwell one to a highly non-linear BI one. Close inspection reveals that this curious and unexpected result can be attributed to the fact that the concrete structure of BI equations happens to be such that it is indistinguishable at the horizon to a local observer, say, in Damour's local tetrad frame from that of standard Maxwell theory.

gr-qc↗

arXiv2001

Electric Charge and Magnetic Flux on Rotating Black Holes in a Force-Free Magnetosphere

The electric charge on rotating black holes is calculated to be ~ BJ in the force-free configuration of Ghosh (2000), with a horizon flux of ~ BM^2. This charge is gravitationally weak for B ~ 10^{15} G, so that the Kerr metric applies. Being similar to the electric charge of a magnetar, both electric charge and magnetic flux should be, in sign and order of magnitude, continuous during stellar collapse into a black hole. Extraction of the rotational energy from newly formed black holes may proceed by interaction with the magnetic field. Keywords:black hole physics --magnetic fields

astro-ph↗

arXiv2001

Toy model for a two-dimensional accretion disk dominated by Poynting flux

We discuss the effect of the Poynting flow on the magnetically dominated thin accretion disk, which is simplified to a two-dimensional disk on the equatorial plane. It is shown in the relativistic formulation that the Poynting flux by the rotating magnetic field with Keplerian angular velocity can balance the energy and angular momentum conservation of a steady accretion flow.

astro-ph↗

arXiv2000

Non-vanishing Magnetic Flux through the Slightly-charged Kerr Black Hole

In association with the Blanford-Znajek mechanism for rotational energy extraction from Kerr black holes, it is of some interest to explore how much of magnetic flux can actually penetrate the horizon at least in idealized situations. For completely uncharged Kerr hole case, it has been known for some time that the magnetic flux gets entirely expelled when the hole is maximally-rotating. In the mean time, it is known that when the rotating hole is immersed in an originally uniform magnetic field surrounded by an ionized interstellar medium (plasma), which is a more realistic situation, the hole accretes certain amount of electric charge. In the present work, it is demonstrated that as a result of this accretion charge small enough not to disturb the geometry, the magnetic flux through this slightly charged Kerr hole depends not only on the hole's angular momentum but on the hole's charge as well such that it never vanishes for any value of the hole's angular momentum.

gr-qc↗

arXiv2000

Evolution of a Rotating Black Hole with a Magnetized Accretion Disk

The effect of an accretion disk on the Blandford-Znajek process and the evolution of a black hole are discussed using a simplified system for the black hole - accretion disk in which the accretion rate is supposed to be dominated by the strong magnetic field on the disk. The evolutions of the mass and the angular momentum of the black hole are formulated and discussed with numerical calculations.

astro-ph↗

arXiv2000

On Companion-Induced Off-Center Supernova-Like Explosions

We suggest that a neutron star with a strong magnetic field, spiraling into the envelope of a companion star, can generate a ``companion induced SN-like off-center explosion". The strongly magnetized neutron star ("magnetar") is born in a supernova explosion before entering into an expanding envelope of a supergiant companion. If the neutron star collapses into a black hole via the hypercritical accretion during the spiral-in phase, a rapidly rotating black hole with a strong magnetic field at the horizon results. The Blandford-Znajek power is sufficient to power a supernova-like event with the center of explosion displaced from the companion core. The companion core, after explosion, evolves into a C/O-white dwarf or a neutron star with a second explosion. The detection of highly eccentric black-hole, C/O-white dwarf binaries or the double explosion structures in the supernova remnants could be an evidence of the proposed scenario.

astro-ph↗

arXiv2000

Issues in the Blandford-Znajek Process for GRB Inner Engine

Several issues regarding the Blandford-Znajek process are discussed to demonstrate that it can be an effective mechanism for powering the gamma ray bursts. Using a simple circuit analysis it is argued that the disk power increases the effective power of the black hole-accretion disk system, although a part of disk power can be dissipated into black hole entropy. Within the framework of the force-free magnetosphere with the strong magnetic field, the magnetically dominated MHD flow is found to support the Blandford-Znajek process and it is demonstrated that the possible magnetic repulsion by the rotating black hole will not affect the efficiency substantially.

astro-ph↗

arXiv1999

Renormalization Group Analysis of ρ-Meson Properties at Finite Density

We calculate the density dependence of the $ρ$-meson mass and coupling constant($g_{ρNN}$) for $ρ$-nucleon-nucleon vertex at one loop using the lagrangian where the $ρ$-meson is included as a dynamical gauge boson of a hidden local symmetry. From the condition that thermodynamic potential should not depend on the arbitrary energy scale, renormalization scale, one can construct a renormalization group equation for the thermodynamic potential and argue that the various renormalization group coefficients are functions of the density or temperature. We calculate the $β$-function for $ρ$-nucleon-nucleon coupling constant ($g_{ρNN}$) and $γ$-function for $ρ$-meson mass ($γ_{m_ρ}$). We found that the $ρ$-meson mass and the coupling constant for $g_{ρNN}$ drop as density increases in the low energy limit.

hep-ph↗

arXiv1999

Blandford-Znajek process as a gamma ray burst central engine

We investigate the possibility that gamma-ray bursts are powered by a central engine consisting of a black hole with an external magnetic field supported by a surrounding disk or torus. The rotational energy of the black hole can be extracted electromagnetically as a Poynting flux, a mechanism proposed by Blandford and Znajek(1977). Recently observed magnetars indicate that some compact objects have very high magnetic fields, up to $10^{15}$ G, which is required to extract the energy within the duration of a GRB, i.e., in 1000 s or less. We demonstrate also that the Poynting flux need not be substantially dominated by the disk.

astro-ph↗