Hongsu Kim

Instanton-meron hybrid in the background of gravitational instantons

In the present work, perhaps the simplest and the most straightforward new algorithm for generating solutions to the (anti-)self-dual Yang-Mills (YM) equation in the typical gravitational instanton background is proposed and then applied to find solutions to practically all the known gravitational instantons. The solutions thus obtained turn out to be some kind of instanton-meron hybrids possessing mixed features of both. Namely, they are rather exotic type configurations obeying a first order (anti-)self-dual YM equation which are everywhere nonsingular and have finite Euclidean YM actions on one hand while exhibiting meronlike large distance behavior and carrying generally fractional topological charge values on the other. Close inspection, however, reveals that the solutions are more like instantons than merons in their generic nature.

Nonvanishing 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 holes angular momentum but on the holes charge as well such that it never vanishes for any value of the holes angular momentum.

Yang-Mills instantons sitting on a Ricci-flat worldspace of double D4-branes

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 D4brane 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 bestknown gravitational instantons. And then we demonstrate that with this YM

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 the 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 the Maxwell theory context and if they do, how. Interestingly enough, as long as one employs the same local null tetrad frame as the one adopted in the works of Damour and of Znajek to read out physical values of electromagnetic fields and a 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 nonlinear 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 Damours local tetrad frame from that of standard Maxwell theory.

Magnetic flux through a slightly charged Kerr black hole

In association with the Blanford-Znajek mechanism for the extraction of rotational energy from a Kerr black hole, it is of interest to explore how much of magnetic flux can actually penetrate the horizon. For the completely uncharged Kerr hole case, it has been known that the magnetic flux gets entirely expelled when the hole is maximally rotating. In the mean time, it is known that a rotating hole immersed in a magnetic field, when it is surrounded by plasma, accretes a certain amount of electric charge. In this work, we show that, as a result of this accretion charge, small enough not to disturb the background geometry, the magnetic flux through this slightly charged Kerr black hole depends not only on the hole’s angular momentum but also on the hole’s charge such that it never vanishes for any value of the hole’s angular momentum.

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