arXiv subjects

Hirotaka Yoshino

Publications and source records attributed to Hirotaka Yoshino.

At least 19 recordsLinked to original sources

arXiv2026

Time evolution of a Nambu-Goto string coiling around a Kerr black hole

The interaction between a Nambu-Goto string and a Kerr black hole gives one of the methods of energy extraction from a rotating black hole. Although the properties of such processes have been well studied for rigidly rotating strings, little is known for non-rigidly rotating strings. In this paper, we study time evolution of a Nambu-Goto string on the equatorial plane of a Kerr spacetime, which sticks on the horizon and extends to spatial infinity. The time evolution is studied by the series expansion with respect to $t$ and the numerical simulations, which give reliable results for $t\lesssim 4M$ and $t\lesssim 38M$, respectively, where $M$ is the black hole mass. Since the angular velocity of the string on the horizon must coincide with the horizon angular velocity to keep the timelike property, the string is dragged into rotation and coils around the black hole. The negative energy is observed to fall into the black hole, but the positive energy follows after that, meaning that the energy extraction occurs for a short period of time. In the outside region, a wave is generated and propagates to the distant region carrying the extracted energy. After the propagation of the wave, the system approaches the time-independent configuration found by Boos and Frolov, and the total extracted energy is estimated as $E_{\rm ext}\lesssim μM$, where $μ$ is the tension of the string.

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arXiv2026

Motion of a charged test particle around a static black hole in a monopole magnetic field

We study the motion of a charged test particle in the spacetime with a spherically symmetric black hole which is immersed in a monopole magnetic field. We show that the radial motion of the charged test particle is governed by completely the same equation as that in the case of no magnetic field. This result implies that the black hole will acquire the electric charge if it is surrounded by the collisionless plasma composed of protons and electrons which obey the Maxwell velocity distribution. The drastically different situation appears in the tangential motions of charged test particles due to the magnetic field. The trajectory of a charged test particle in the black hole with the magnetic field of the order of 10 Gauss near the black hole, is confined on a very thin cone as long as the specific angular momentum of the particle is not much larger than the gravitational radius of the black hole times the speed of light. This result leads to a possibility that a plasma lump can hover over the black hole and is very hot, in the monopole magnetic field.

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arXiv2025

Quasilocal inequalities for attractive gravity probe surface

We discuss the local and quasilocal properties of the loosely trapped surface (LTS) and the attractive gravity probe surface (AGPS), which have been proposed to characterize the strength of gravity in both strong and weak gravity regions using the mean curvature. In terms of local mass defined in a region surrounded by the two AGPSs and of Geroch quasilocal mass, we present several inequalities concerning their size and area, which are of particular interest. We also propose the improved concepts of the LTS/AGPS, which we call LTS Plus (LTS+) and AGPS Plus (AGPS$+$), defined in terms of expansions of outgoing and ingoing null geodesic congruences on those surfaces. Then, the similar inequalities are proven in terms of appropriately defined local mass and the Hawking quasilocal mass.

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arXiv2025

Classification of global structures of evaporating regular black holes with infinite periods of time

We carry out model independent analyses for global structures of spherically symmetric regular black holes that evaporate and approach the extremal state spending infinite periods of time due to Hawking radiation. We assume the radius of the outer apparent horizon (outer AH) to be a decreasing function of ingoing null coordinate, and consider the three cases, Cases 1, 2, and 3, where the radius of the inner AH is a constant, increases, and decreases, respectively. A complete classification of the Penrose diagrams is presented in each case, taking account not only of the presence and absence of the event horizon but also of the relative positions of the inner and outer AHs. Sufficient conditions that lead to each type are proved, and the examples of each type are summarized. We also study the behavior of outgoing null geodesics in models where the null geodesic equation is solvable, and figure out the important factors to determine the type of a spacetime using concepts in the area of dynamical systems. The formation of a Cauchy horizon is also established in the solvable models when the event horizon is present.

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arXiv2025

Attractive gravity probe surface in Einstein-Maxwell system

We derive areal inequalities for five types of attractive gravity probe surfaces, which were proposed by us in order to characterize the strength of gravity in different ways including weak gravity region, taking into account of contributions of electric and magnetic charges, angular momentum, gravitational waves, and matters. These inequalities are generalizations of the Riemannian Penrose inequality for minimal surfaces, and lead to the concept of extremality for a given surface whose condition is given in terms of the gravitational mass and the electromagnetic charges. This means that the extremality is a characteristic property not only of black hole horizons or minimal surfaces but also of surfaces in weak gravity region. We also derive areal inequalities and extremality conditions for surfaces in asymptotically locally anti-de Sitter spacetimes.

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arXiv2025

Initial data for a black string and a Kaluza-Klein bubble: Space-dependent compactification radius

As the first step to explore the nonlinear dynamics of an extra dimension in the Kaluza-Klein (KK) spacetime with black objects through numerical relativity, we generate time-symmetric initial data of a black string and/or a KK bubble with space-dependent compactification radius. The initial data developed in this paper are classified into three types. First, we present analytic initial data with SO(3) symmetry whose three-dimensional section is spherically symmetric. These initial data include a black string without a KK bubble, a black string trapping a KK bubble, and a naked KK bubble. Second, we present analytic initial data for multiple black strings with varying compactification radius, which is a natural generalization of the Brill-Lindquist initial data for four-dimensional general relativity. Finally, we develop a numerical method for generating the initial data with a black string and a KK bubble located at different positions, which would be useful in simulating what happens when an expanding KK bubble meets black objects in dynamical context.

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arXiv2024

Electrification of a non-rotating black hole

Zajacek et al made an interesting theoretical prediction on the electrification of a non-rotating black hole; if a non-rotating black hole is surrounded by plasma composed of protons and electrons, it will acquire electric charge due to the large difference of the inertial mass of a proton and that of an electron. Furthermore they revealed the effects of the electric charge of the black hole on the surrounding plasma. Since their results mainly rely on non-relativistic analyses, we study the same subject through relativistic analyses in this paper. By investigating a test particle in the Schwarzschild spacetime, we find that if initial velocities of protons and electrons far from a black hole follow the Maxwell distribution, the black hole can acquire electric charge whose value depends on the ratio of temperature of the proton and that of the electron. We also show that if the black hole acquires the electric charge, the radii of the innermost stable circular orbit (ISCO) and the specific energy of a charged particle on ISCO can be very different from those of a neutral particle. In contrast to the result obtained by Zajacek et al, we find that the ISCO radii of a proton and an electron are necessarily larger than that of a neutral test particle as long as the black hole acquires the charge. The large ISCO radius might lead to a larger angular diameter of a black hole shadow and a different estimate of the released energy due to the accretion of plasma from the estimate based on the assumption of electric neutrality of the central black hole.

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arXiv2024

Loosely trapped surface for slowly rotating black hole

We construct the marginal loosely trapped surface (marginal LTS) for the Kerr spacetime with a small Kerr parameter perturbatively, where the LTS condition is saturated. An LTS is a surface that specifies the strong gravity region, which is a generalization of the photon sphere in the Schwarzschild spacetime. It turns out that there are an infinite number of marginal LTSs. At the leading order of the small Kerr parameter, all of the marginal LTSs have the same area. However, one can see that the maximal marginal LTS among them is uniquely determined at the higher order.

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arXiv2024

A generalization of photon sphere based on escape/capture cone

In asymptotically flat spacetimes, bearing the null geodesics reaching the future null infinity in mind, we propose new concepts, the "dark horizons" as generalizations of the photon sphere. They are defined in terms of the structure of escape/capture cones of photons with respect to a unit timelike vector field. More specifically, considering a two-sphere that represents a set of emission directions of photons, the dark horizons are located at positions where a hemisphere is marginally included in the capture and escape cones, respectively. We show that both of them are absent in the Minkowski spacetime, while they exist in spacetimes with black hole(s) under a certain condition. We derive the general properties of the dark horizons in spherically symmetric spacetimes and explicitly calculate the locations of the dark horizons in the Vaidya spacetime and the Kerr spacetime.

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arXiv2024

Deci-Hz gravitational waves from the self-interacting axion cloud around the rotating stellar mass black hole

Gravitational waves from condensates of ultra-light particles, such as axion, around rotating black holes are a promising probe to search for unknown physics. For this purpose, we need to characterize the signal to detect the gravitational waves, which requires tracking the evolution of the condensates, including various effects. The axion self-interaction causes the non-linear coupling between the superradiant modes, resulting in complicated branching of evolution. Most studies so far have considered evolution under the non-relativistic approximation or the two-mode approximation. In this paper, we numerically investigate the evolution of the axion condensate without these approximations, taking higher multipole modes into account. We also investigate the possible signature in gravitational waves from the condensate. We show that the higher multipole modes are excited, leading to the gravitational wave signal by the transition of the axion between different levels. The most prominent signal of gravitational waves arises from the transition between modes with their angular quantum numbers different by two. The gravitational wave signal is emitted in the deci-Hz band for stellar mass black holes, which might be observable with the future gravitational wave detectors.

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arXiv2024

Evaporation of a nonsingular Reissner-Nordström black hole and information loss problem

One of the attractive solutions to the information loss problem is that the event horizon does not appear in the process of gravitational collapse and subsequent evaporation once the spacetime singularity is regularized by some mechanism, as pointed out by Hayward and Frolov. In this paper, we examine whether this Hayward-Frolov scenario holds for the evaporation of a charged black hole. The process of collapse and evaporation is modeled with the charged Vaidya spacetime and two kinds of regularization of the central singularity are considered. Analyzing the spacetime structure of the evaporating black hole, we find that the appropriately regularized evaporating Reissner-Nordström "black hole" has no event and Cauchy horizons, indicating the possibility that the Hayward-Frolov scenario may have sufficient generality as the solution to the information loss problem. In addition, the properties of the non-singular evaporating Reissner-Nordstoöm black hole are examined in detail.

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arXiv2023

Distorted static photon surfaces in perturbed Reissner-Nordström spacetimes

The photon surface is defined as a timelike surface $S$ such that any photon emitted in arbitrary tangential direction to $S$ from an arbitrary point on $S$ continues to propagate on $S$. In this paper, we examine whether a static photon surface can be present in distorted electrovacuum spacetimes with perturbative approach, by constructing analytic solutions to the equations for static perturbations of a Reissner-Nordström spacetime that are regular outside the background photon surface. For each of the $\ell\ge 2$ modes, there are two physical solutions to the perturbative equations that correspond to adding the multipole moments of mass and electric charge, respectively. By adjusting the ratio of the amplitudes of the two solutions appropriately, it is possible to realize a distorted photon surface. In the case of $\ell=1$, although there is only one physical solution to the perturbative equations, there is also a degree of freedom to shift the coordinate position of the photon surface, and this enables the formation of a photon surface. Such a photon surface has a spherically symmetric spatial section, but it is also distorted in the three-dimensional sense. Our results show that distorted static photon surfaces can be formed at least at the level of the first-order perturbations, and imply that the uniqueness of the photon surface may not hold in electrovacuum spacetimes.

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arXiv2023

Asymptotic behavior of null geodesics near future null infinity IV: Null-access theorem for generic asymptotically flat spacetime

In our previous papers [arXiv:2106.03150, arXiv:2110.10917, arXiv:2208.00822], we analyzed the asymptotic behavior of future directed null geodesics near future null infinity and then we showed a proposition on the accessibility of the null geodesics to future null infinity in a specific class of asymptotically flat spacetimes. In this paper, we adopt the retarded time of the Bondi coordinate as the parameter for the null geodesics and then see that one can relax the assumptions imposed in our previous studies. As a consequence, we obtain a new null-access theorem for generic asymptotically flat spacetimes.

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arXiv2022

Impact of multiple modes on the evolution of self-interacting axion condensate around rotating black holes

Ultra-light particles, such as axions, form a macroscopic condensate around a highly spinning black hole by the superradiant instability. Due to its macroscopic nature, the condensate opens the possibility of detecting the axion through gravitational wave observations. However, the precise evolution of the condensate must be known for the actual detection. For future observation, we numerically study the influence of the self-interaction, especially interaction between different modes, on the evolution of the condensate in detail. First, we focus on the case when condensate starts with the smallest possible angular quantum number. For this case, we perform the non-linear calculation and show that the dissipation induced by the mode interaction is strong enough to saturate the superradiant instability, even if the secondary cloud starts with quantum fluctuations. Our result indicates that explosive phenomena such as bosenova do not occur in this case. We also show that the condensate settles to a quasi-stationary state mainly composed of two modes, one with the smallest angular quantum number for which the superradiant instability occurs and the other with the adjacent higher angular quantum number. We also study the case when the condensate starts with the dominance of the higher angular quantum number. We show that the dissipation process induced by the mode coupling does not occur for small gravitational coupling. Therefore, bosenova might occur in this case.

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arXiv2022

Asymptotic behavior of null geodesics near future null infinity. III. Photons towards inward directions

A new sufficient condition for photons emitted near future null infinity to reach future null infinity is derived by studying null geodesics in the Bondi coordinates in asymptotically flat spacetimes. In our previous works [arXiv:2106.03150, arXiv:2110.10917], such a condition was established for photons emitted in outward or tangential directions to constant radial surfaces. This paper improves our previous result by including photons emitted in inward directions. In four dimensions, imposing the same assumptions on the metric functions as previously, we prove that photons reach future null infinity if their initial values of $|dr/du|$ are smaller than a certain quantity, where $r$ and $u$ are the radial and retarded time coordinates, respectively. This quantity is determined by the asymptotic properties of the metric and is connected to the conjectured maximal luminosity. In higher dimensions, photons emitted with $dr/du>-(1-1/\sqrt{3})\approx -0.423$ are shown to reach future null infinity without the assumptions on the metric functions.

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arXiv2022

Attractive gravity probe surfaces in higher dimensions

A generalization of the Riemannian Penrose inequality in $n$-dimensional space ($3\le n<8$) is done. We introduce a parameter $α$ ($-\frac{1}{n-1}<α< \infty$) indicating the strength of the gravitational field, and define a refined attractive gravity probe surface (refined AGPS) with $α$. Then, we show the area inequality for a refined AGPS, $A \le ω_{n-1} \left[ (n+2(n-1)α)Gm /(1+(n-1)α) \right]^{\frac{n-1}{n-2}}$, where $A$ is the area of the refined AGPS, $ω_{n-1}$ is the area of the standard unit $(n-1)$-sphere, $G$ is Newton's gravitational constant and $m$ is the Arnowitt-Deser-Misner mass. The obtained inequality is applicable not only to surfaces in strong gravity regions such as a minimal surface (corresponding to the limit $α\to \infty$), but also to those in weak gravity existing near infinity (corresponding to the limit $α\to -\frac{1}{n-1}$).

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arXiv2022

Four types of attractive gravity probe surfaces

We reexamine the concept of the attractive gravity probe surface recently proposed as an indicator for strength of gravity. Then, we propose three new variant concepts and show refined inequalities for the four types of the AGPSs by taking account of the angular momentum, gravitational waves and matters.

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arXiv2022

Asymptotic behavior of null geodesics near future null infinity II: curvatures, photon surface and dynamically transversely trapping surface

Bearing in mind our previous study on asymptotic behavior of null geodesics near future null infinity, we analyze the behavior of geometrical quantities such as a certain extrinsic curvature and Riemann tensor in the Bondi coordinates. In the sense of asymptotics, the condition for an $r$-constant hypersurface to be a photon surface is shown to be controlled by a key quantity that determines the fate of photons initially emitted in angular directions. As a consequence, in four dimensions, such a non-expanding photon surface can be realized even near future null infinity in the presence of enormous energy flux for a short period of time. By contrast, in higher-dimensional cases, no such a photon surface can exist. This result also implies that the dynamically transversely trapping surface, which is proposed as an extension of a photon surface, can have an arbitrarily large radius in four dimensions.

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