arXiv subjects

Isao Okamoto

Publications and source records attributed to Isao Okamoto.

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↗

arXiv2005

Electromagnetic extraction of energy from Kerr black holes

We elucidate the `right' process for energy extraction from Kerr black holes through `FFDE' magnetospheres, free from causality violation. It is shown that the magnetosphere of a Kerr black hole possesses the double-structure, consisting of the inner and outer magnetospheres with the pair-creation gap between them and with respective unipolar batteries at the inner and outer surfaces of the gap.

astro-ph↗

arXiv2000

Fluctuations in Isothermal Spheres

Isolated isothermal spheres of N gravitationally interacting points with equal mass are believed to be stable when density contrasts do not exceed 709. That stability limit does, however, not take into consideration fluctuations of temperature near the onset of instability. These are important when N is finite. Here we correlate {\it global mean quadratic temperature fluctuations} with onset of instability. We show that such fluctuations trigger instability when the density contrast reaches a value near $709\cdot\exp(-3.3N^{-1/3})$. These lower values of limiting density contrasts are significantly smaller than 709 when N is not very big and this suggests (i) that numerical calculations with small N may not reflect correctly the onset of core collapse in clusters with big N and (ii) that a greater number of globular clusters than is normally believed may already be in an advanced stage of core collapse because most of observed globular clusters whose parameters fit quasi-isothermal configurations are close to marginal stability.

astro-ph↗

arXiv1994

Thermodynamics of a black hole in a cavity

We present a unified thermodynamical description of the configurations consisting on self-gravitating radiation with or without a black hole. We compute the thermal fluctuations and evaluate where will they induce a transition from metastable configurations towards stable ones. We show that the probability of finding such a transition is exponentially small. This indicates that, in a sequence of quasi equilibrium configurations, the system will remain in the metastable states till it approaches very closely the critical point beyond which no metastable configuration exists. Near that point, we relate the divergence of the local temperature fluctuations to the approach of the instability of the whole system, thereby generalizing the usual fluctuations analysis in the cases where long range forces are present. When angular momentum is added to the cavity, the above picture is slightly modified. Nevertheless, at high angular momentum, the black hole loses most of its mass before it reaches the critical point at which it evaporates completely.

gr-qc↗