arXiv subjects

Hongsu Kim

Publications and source records attributed to Hongsu Kim.

At least 37 records · Page 2Linked to original sources

arXiv2005

Bonnor-type Black Dihole Solution in Brans-Dicke-Maxwell Theory

It was originally thought that Bonnor's solution in Einstein-Maxwell theory describes a singular point-like magnetic dipole. Lately, however, it has been demonstrated that indeed it may describe a black {\it dihole}, i.e., a pair of static, oppositely-charged extremal black holes with regular horizons. Motivated particularly by this new interpretation, in the present work, the construction and extensive analysis of a solution in the context of the Brans-Dicke-Maxwell theory representing a black dihole are attempted. It has been known for some time that the solution-generating algorithm of Singh and Rai produces stationary, axisymmetric, charged solutions in Brans-Dicke-Maxwell theory from the known such solutions in Einstein-Maxwell theory. Thus this algorithm of Singh and Rai's is employed in order to construct a Bonnor-type magnetic black dihole solution in Brans-Dicke-Maxwell theory from the known Bonnor solution in Einstein-Maxwell theory. The peculiar feature of the new solution including internal infinity nature of the symmetry axis and its stability issue have been discussed in full detail.

gr-qc↗

arXiv2005

Pulsar Magnetosphere : A General Relativistic Treatment

A fully general relativistic description of the pulsar magnetosphere is provided. To be more concrete, a study of the pulsar magnetosphere is performed in the context of general relativistic magnetohydrodynamics (MHD) employing the so-called Grad-Shafranov approach. Not surprisingly, the resulting Grad-Shafranov equations and all the other related general relativistic MHD equations turn out to take essentially the same structures as those for the (rotating) black hole magnetosphere. Other different natures between the two cases including the structure of singular surfaces of MHD flows in each magnetosphere are essentially encoded in the different spacetime (metric) contents. In this way, the pulsar and the black hole magnetospheres can be described in an unified fashion. Particularly, the direction of poloidal currents circulating in the neutron star magnetosphere turns out to be the same as that of currents circulating in the black hole magnetosphere which, in turn, leads to the pulsar and the black hole spin-downs via the ``magnetic braking''.

astro-ph↗

arXiv2004

Brans-Dicke Scalar Field as a Unique k-essence

In the present work, the Brans-Dicke theory of gravity is taken as a possible theory of k-essence coupled to gravity and then the role played by the Brans-Dicke scalar field in relation to the unified model for dark matter and dark energy is explored. It has been realized that the BD scalar field does indeed play a role of a k-essence, but in a very unique way which distinguishes it from other k-essence fields studied in the literature such as the (generalized) Chaplygin gas or the purely kinetic k-essence studied recently by Scherrer. That is, instead of providing an interpolation between the dark matter-like evolution and the dark energy-like evolution, the BD theory predicts the emergence of a yet-unknown {\it zero acceleration} epoch which is an intermediate stage acting as a ``crossing bridge'' between the decelerating matter-dominated era and the accelerating phase.

astro-ph↗

arXiv2004

Quantum Description for the Decay of NSNS Brane-Antibrane Systems

The stringy description for the instabilities in the $RR$ charged $D_{p}-\bar{D}_{p}$ pairs is now well understood in terms of the open string tachyon condensation. The quantum interpretation presumably via the stringy description for the instabilities in the $NSNS$-charged $F1-\bar{F1}$ and $NS5-\bar{NS5}$ pairs in IIA/IIB theories, however, has not been established yet. This would be partly because of the absence (for the $F1-\bar{F1}$ case) or our relatively poor understanding (for the $NS5-\bar{NS5}$ case) of their worldvolume (gauge theory) dynamics. In the present work, using the well-known quantum description for instabilities in the $RR$-charged $D_{p}-\bar{D}_{p}$ systems and in the M-theory brane-antibrane systems and invoking appropriate string dualities, the stringy nature of the instabilities in the $NSNS$-charged $F1-\bar{F1}$ and $NS5-\bar{NS5}$ systems has been uncovered. For the annihilations to string vacua, the quantum, stringy interpretations are simple extensions of Sen's conjecture for those in $RR$-charged brane-antibrane systems.

hep-th↗

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↗

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

Supergravity Duals to the Noncommutative N=4 SYM theory in the Infinite Momentum Frame

In this work the construction of supergravity duals to the noncommutative ${\cal N}=4$ SYM theory in the infinite momentum frame but with constant momentum density is attempted. In the absence of the content of noncommutativity, it has been known for some time that the previous $AdS_{5}/CFT_{4}$ correspondence should be replaced by the $K_{5}/CFT_{4}$ (with $K_{(p+2)}$ denoting the generalized Kaigorodov spacetime) correspondence with the pp-wave propagating on the BPS brane worldvolume. Interestingly enough, putting together the two contents, i.e., the introduction of noncommutativity and at the same time that of the pp-wave along the brane worldvolume, leads to quite nontrivial consequences such as the emergence of ``time-space'' noncommutativity in addition to the ``space-space'' noncommutativity in the manifold on which the dual gauge theory is defined. Taking the gravity decoupling limit, it has been realized that for small $u$, the solutions all reduce to $K_{5}\times S^5$ geometry confirming our expectation that the IR dynamics of the dual gauge theory should be unaffected by the noncommutativity while as $u\to \infty$, the solutions start to deviate significantly from $K_{5}\times S^5$ limit indicating that the UV dynamics of the dual gauge theory would be heavily distorted by the effect of noncommutativity.

hep-th↗

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↗

arXiv2003

Supergravity Approach to Tachyon Potential in Brane-Antibrane Systems

Using an exact supergravity solution representing the Dp-\bar{Dp} system, it is demonstrated that one can construct a supergravity analogue of the tachyon potential. Remarkably, the (regularized) minimum value of the potential turns out to be V(T_{0})=-2m with m denoting the ADM mass of a single Dp-brane. This result, in a sense, appears to confirm that Sen's conjecture for the tachyon condensation on unstable D-branes is indeed correct although the analysis used here is semi-classical in nature and hence should be taken with some care. Also shown is the fact that the tachyon mass squared m^2_{T} (which has started out as being negative) can actually become positive definite and large as the tachyon rolls down toward the minimum of its potential. It indeed signals the possibility of successful condensation of the tachyon since it shows that near the minimum of its potential, tachyon can become heavy enough to disappear from the massless spectrum. Some cosmological implications of this tachyon potential in the context of ``rolling tachyons'' is also discussed.

hep-th↗

arXiv2002

Fluxbranes Delaying Brane-Antibrane Annihilation

By intersecting the RR charged D_p - bar{D}_p pair (p=6, 4, 2, 0) with the RR F7-brane and by intersecting the NSNS charged F1-bar{F1} and NS5-bar{NS5} pairs with the NSNS F6-branes, the possibility of stabilizing the brane-antibrane systems is considered. The behavior of the corresponding supergravity solutions indicates that the RR F7-brane content of the solution plays the role of keeping the brane and the antibrane from annihilating each other completely since the two-brane configuration structure still persists in the vanishing inter-brane distance limit of the supergravity solution. In terms of the stringy description, we interpret this as representing that the RR F7-brane ``delays'' the brane-antibrane annihilation process but only until this non-supersymmetric and hence unstable F7-brane itself decays. Then next, the behavior of the supergravity solutions representing F1-bar{F1} and NS5-bar{NS5} again for vanishing inter-brane separation reveals that as they approach, these ``NS''-charged brane and antibrane always collide and annihilate irrespective of the presence or the absence of the NSNS F6-brane. And we have essentially attributed this to the absence of (open) stringy description of the instability in the ``NS''-charged case. This interpretation may provide a resolution to the contrasting features between the instability of ``R''-charged brane-antibrane systems and that of `NS''-charged ones. Certainly, however, it poses another puzzle that in the ``NS''-charged case, the quantum entity, that should take over the semi-classical instability as the inter-brane distance gets smaller, is missing. This is rather an embarrassing state of affair that needs to be treated with great care.

hep-th↗

arXiv2002

Removal of Closed Timelike Curves in Kerr-Newman Spacetime

A simple yet systematic new algorithm to investigate the global structure of Kerr-Newman spacetime is suggested. Namely, the global structure of θ=const. timelike submanifolds of Kerr-Newman metric are studied by introducing a new time coordinate slightly different from the usual Boyer-Lindquist time coordinate. In addition, it is demonstrated that the possible causality violation thus far regarded to occur near the ring singularity via the development of closed timelike curves there is not really an unavoidable pathology which has plagued the Kerr-Newman solution but simply a gauge (coordinate) artifact as it disappears upon transforming from Boyer-Lindquist to the new time coordinate. This last point appears to lend support to the fact that, indeed, the Kerr-Newman spacetime is a legitimate solution to represent the interior as well as the exterior regions of a rotating, charged black hole spacetime.

gr-qc↗

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

Yang-Mills Instantons Sitting on a Ricci-flat Worldspace of Double D4-brane

Thus far, there seem to be no complete criteria that can settle the issue as to what the correct generalization of the Dirac-Born-Infeld (DBI) action, describing the low-energy dynamics of the D-branes, to the non-abelian case would be. According to recent suggestions, one might pass the issue of worldvolume solitons from abelian to non-abelian setting by considering the stack of multiple, coincident D-branes and use it as a guideline to construct or censor the relevant non-abelian version of the DBI action. In this spirit, here we are interested in the explicit construction of SU(2) Yang-Mills (YM) instanton solutions in the background geometry of two coincident probe D4-brane worldspaces particularly when the metric of target spacetime in which the probe branes are embedded is given by the Ricci-flat, magnetic extremal 4-brane solution in type IIA supergravity theory with its worldspace metric being given by that of Taub-NUT and Eguchi-Hanson solutions, the two best-known gravitational instantons. And then we demonstrate that with this YM instanton- gravitational instanton configuration on the probe D4-brane worldvolume, the energy of the probe branes attains its minimum value and hence enjoys stable state provided one employs the Tseytlin's non-abelian DBI action for the description of multiple probe D-branes. In this way, we support the arguments in the literature in favor of Tseytlin's proposal for the non-abelian DBI action.

hep-th↗

arXiv2000

Instanton-Meron Hybrid in the Background of Gravitational Instantons

When it comes to the topological aspects, gravity may have profound effects even at the level of particle physics despite its negligibly small relative strength well below the Planck scale. In spite of this intriguing possibility, relatively little attempt has been made toward the exhibition of this phenomenon in relevant physical systems. In the present work, perhaps the simplest and the most straightforward new algorithm for generating solutions to (anti) self-dual Yang-Mills (YM) equation in the typical gravitational instanton backgrounds is proposed and then applied to find the solutions practically in all the gravitational instantons known. Solutions thus obtained turn out to be some kind of instanton-meron hybrids possessing mixed features of both. Namely, they are rather exotic type of configurations obeying first order (anti) self-dual YM equation which are everywhere non-singular and have finite Euclidean YM actions on one hand while exhibiting meron-like large distance behavior and carrying generally fractional topological charge values on the other. Close inspection, however, reveals that the solutions are more like instantons rather than merons in their generic natures.

hep-th↗

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

Yang-Mills Instantons in the Gravitational Instanton Backgrounds

The simplest and the most straightforward new algorithm for generating solutions to (anti) self-dual Yang-Mills (YM) equation in the typical gravitational instanton backgrounds is proposed. When applied to the Taub-NUT and the Eguchi-Hanson metrics, the two best-known gravitational instantons, the solutions turn out to be the rather exotic type of instanton configurations carrying finite YM action but generally fractional topological charge values.

hep-th↗

arXiv1999

Genuine Dyons in Born-Infeld Electrodynamics

Study of magnetic monopoles in the original version of Born-Infeld (BI) electrodynamics is performed. It then is realized that interesting new physics emerge and they include exotic behavior of radial electric monopole field such as its regularity as $r\to 0$ and its changing behavior with the absence or presence of the radial magnetic monopole field. This last point has been interpreted as the manifestation of the existence of point-like dyons in abelian BI theory. Two pieces of clear evidences in favor of this dyon interpretation are provided. It is also demonstrated that despite these unique features having no analogues in standard Maxwell theory, the cherished Dirac quantisation condition remains unchanged. Lastly, comments are given concerning that dyons found here in the original version of BI electrodynamics should be distinguished from the ones with the same name or BIons being studied in the recent literature on D-brane physics.

hep-th↗

arXiv1999

Quantisation of Conformal Fields in Three-dimensional Anti-de Sitter Black Hole Spacetime

Utilizing the conformal-flatness nature of 3-dim. Anti-de Sitter (AdS_3) black hole solution of Banados, Teitelboim and Zanelli, the quantisation of conformally-coupled scalar and spinor fields in this background spacetime is explicitly carried out. In particular, mode expansion forms and propagators of the fields are obtained in closed forms. The vacuum in this conformally-coupled field theories in AdS_3 black hole spacetime, which is conformally-flat, is the conformal vacuum which is unique and has global meaning. This point particularly suggests that now the particle production by AdS_3 black hole spacetime should be absent. General argument establishing the absence of real particle creation by AdS_3 black hole spacetime for this case of conformal triviality is provided. Then next, using the explicit mode expansion forms for conformally-coupled scalar and spinor fields, the bosonic and fermionic superradiances are examined and found to be absent confirming the expectation.

hep-th↗