arXiv subjects

Yoogeun Song

Publications and source records attributed to Yoogeun Song.

5 recordsLinked to original sources

arXiv2024

Electromagnetic Energy Extraction in Kerr Black Holes through Frame-Dragging Magnetospheres

It is argued that the zero-angular-momentum-observers (ZAMOs) circulating with the frame-dragging-angular-velocity $ω$ plays a leading part in energy extraction. When the condition $Ω_{\rm F}<Ω_{\rm H}$ is satisfied, where $Ω_{\rm H}$ and $Ω_{\rm F}$ are the horizon and field-line (FL) angular-velocities (AVs), they will see that the null surface S$_{\rm N}$ with $ω_{\rm N}=Ω_{\rm F}$ always exists in the force-free magnetosphere. The pivotal ZAMO-measured FLAV $Ω_{\rm ZF} \equiv Ω_{\rm F} - ω$ changes sign on this surface S$_{\rm N}$ where the force-free and freezing-in conditions break down. The force-free magnetosphere is divided on this surface, with particle-current sources on it. The outer domain ${D}_{\rm (out)}$ outside S$_{\rm N}$ spins forward ($Ω_{\rm ZF}>0$), whereas the inner domain ${D}_{\rm (in)}$ inside spins backward ($Ω_{\rm ZF}<0$). Because the electric field ${\bf E}_{\rm p}$ reverses direction there, the Poynting flux reverses direction as well from outward to inward, though the positive angular momentum always flows outwardly. Electromagnetic self-extraction of energy will be possible only through the frame-dragged magnetosphere, with the inner domain ${D}_{\rm (in)}$ nested between the horizon and the surface S$_{\rm N}$, when $Ω_{\rm F}<Ω_{\rm H}$ is ensured by the 1st and 2nd laws of thermodynamics.

astro-ph.GA↗

arXiv2023

Energy self-extraction of a Kerr black hole through its frame-dragged force-free magnetosphere

It is shown that when only the condition} $0<Ω_{\rm F}<Ω_{\rm H}$ is satisfied, the Kerr black hole frame-drags its surrounding force-free magnetosphere with the field-line-angular-velocity (FLAV) $Ω_{\rm F}$, where $Ω_{\rm H}$ is the horizon angular-velocity. Then, the zero-angular-momentum-observers (ZAMOs) circulating with the frame-dragging-angular-velocity $ω$ will see that the `null surface' S$_{\rm N}$ where $ω_{\rm N}=Ω_{\rm F}$ always exists. They will see that the outer domain $D_{\rm (out)}$ outside S$_{\rm N}$ is prograde-rotating with $Ω_{{\rm F} ω}>0$, whereas the inner domain $D_{\rm (in)}$ inside is retrograde-rotating with $Ω_{{\rm F} ω}<0$, where $Ω_{{\rm F} ω}=Ω_{\rm F} - ω$ denotes the ZAMO-FLAV. `This surface' S$_{\rm N}$ must be the magneto-centrifugal divider of the force-free magnetosphere, with a kind of plasma-shed on it. Subsequently, the force-free and freezing-in conditions break down on S$_{\rm N}$, thereby allowing the particle-current sources to be set up on S$_{\rm N}$. `This surface' also is the ZAM-surface S$_{\rm ZAMD}$, on which no flow of angular momentum nor electric current can cross. Because the electric field ${\bf E}_{\rm p}$ reverses sign on S$_{\rm N}$, the Poynting flux reverses direction from outward to inward on S$_{\rm N}$. An electromagnetic self-extraction of energy will be possible only through the frame-dragged magnetosphere, with the inner domain $D_{\rm (in)}$ nested between the horizon and `this surface' S$_{\rm N}$, in order to comply with the first and second laws of thermodynamics.

gr-qc↗

arXiv2018

High-energy and very-high-energy emission from stellar-mass black holes moving in gaseous clouds

We investigate the electron-positron pair cascade taking place in the magnetosphere of a rapidly rotating black hole. Because of the spacetime frame dragging, the Goldreich-Julian charge density changes sign in the vicinity of the event horizon, which leads to an occurrence of a magnetic-field aligned electric field, in the same way as the pulsar outer-magnetospheric accelerator. In this lepton accelerator, electrons and positrons are accelerated in the opposite directions, to emit copious gamma-rays via the curvature and inverse-Compton processes. We examine a stationary pair cascade, and show that a stellar-mass black hole moving in a gaseous cloud can emit a detectable very-high-energy flux, provided that the black hole is extremely rotating and that the distance is less than about 1 kpc. We argue that the gamma-ray image will have a point-like morphology, and demonstrate that their gamma-ray spectra have a broad peak around 0.01-1 GeV and a sharp peak around 0.1 TeV, that the accelerators become most luminous when the mass accretion rate becomes about 0.01% of the Eddington rate, and that the predicted gamma-ray flux little changes in a wide range of magnetospheric currents. An implication of the stability of such a stationary gap is discussed.

astro-ph.HE↗

arXiv2017

Enhanced gamma radiation toward the rotation axis from the immediate vicinity of extremely rotating black holes

We investigate the acceleration of electrons and positrons by magnetic-field-aligned electric fields in the polar funnel of an accreting black hole (BH). Applying the pulsar outer-gap theory to BH magnetospheres, we find that such a lepton accelerator arises in the immediate vicinity of the event horizon due to frame-dragging, and that their gamma-ray luminosity increases with decreasing accretion rate. Furthermore, we demonstrate that the gamma-ray flux is enhanced along the rotation axis by more than an order of magnitude if the BH spin increases from $a=0.90M$ to $a=0.9999M$. As a result, if a ten-solar-mass, almost-maximally rotating BH is located within 3 kpc, when its accretion rate is between 0.005% and 0.01% of the Eddington rate, its high-energy flare becomes detectable with the Fermi/Large Area Telescope, provided that the flare lasts longer than 1.2 months and that we view the source nearly along the rotation axis. In addition, its very-high-energy flux is marginally detectable with the Cherenkov Telescope Array, provided that the flare lasts longer than a night and that our viewing angle is about 45 degrees with respect to the rotation axis.

astro-ph.GA↗

arXiv2016

The Influence of the Shear on the Gravitational Waves in the Early Anisotropic Universe

We study the singularity of the congruences for both timelike and null geodesic curves using the expansion of the early anisotropic Bianchi type I Universe. In this paper, we concentrate on the influence of the shear of the timelike and null geodesic congruences in the early Universe. Under some natural conditions, we derive the Raychaudhuri type equation for the expansion and the shear-related equations. Recently, scientists working on the LIGO (Laser Interferometer Gravitational-Wave Observatory) have shown many possibilities to observing the anisotropy of the primordial gravitational wave background radiation. We deduce the evolution equation for the shear that may be responsible for those observational results.

gr-qc↗