189 citations · 439 across the 19 of their papers we have counts for
19 papers
Odd-parity ringdown gravitational waves of a spherically symmetric black hole with perfect fluid accretion
Rikuto Ohashi, Yasutaka Koga, Hiroyuki Nakano +2
The ringdown waves from a black hole offer a clean probe of strong-field gravity, but a matter distribution that may be present around a realistic black hole renders the background…
Test particle motion around a black hole dressed with a spherically symmetric stationary fluid
Ariadna Uxue Palomino Ylla, Yasutaka Koga, Chul-Moon Yoo
We investigate the motion of a massive particle around a spherically symmetric black hole surrounded by a stationary and radial inflow of perfect fluid. The background spacetime is…
Shadow formation in gravitational collapse: Redshift and blueshift by spacetime dynamics
Yasutaka Koga, Nobuyuki Asaka, Masashi Kimura +1
A black hole illuminated by a background light source is observed as a black hole shadow. For a black hole formed by gravitational collapse of a transmissive object, redshift of li…
AdS gravastar and bulk-cone singularities
Heng-Yu Chen, Yasuaki Hikida, Yasutaka Koga
The horizon of black hole is surrounded by the photon sphere and an outside observer cannot easily examine the geometry inside the photon sphere. In this note, we propose a way to…
Geometrical origin for the compaction function for primordial black hole formation
Tomohiro Harada, Hayami Iizuka, Yasutaka Koga +1
We propose a geometrical origin for the Shibata-Sasaki compaction function, which is known to be a reliable indicator of primordial black hole formation at least during radiation d…
Revisiting spins of primordial black holes in a matter-dominated era based on peak theory
Daiki Saito, Tomohiro Harada, Yasutaka Koga +1
We estimate the probability distribution for the spins of the primordial black holes (PBHs) that formed during an early matter-dominated era in the Universe. We employ the Zel'dovi…
Spins of primordial black holes formed with a soft equation of state
Daiki Saito, Tomohiro Harada, Yasutaka Koga +1
We investigate the probability distribution of the spins of primordial black holes (PBHs) formed in the universe dominated by a perfect fluid with the linear equation of state $p=w…
Revisiting compaction functions for primordial black hole formation
Tomohiro Harada, Chul-Moon Yoo, Yasutaka Koga
Shibata and Sasaki [arXiv:gr-qc/9905064] introduced the so-called compaction function. Since then, it has been empirically established that the maximum value of this function (or i…
Effective inspiral spin distribution of primordial black hole binaries
Yasutaka Koga, Tomohiro Harada, Yuichiro Tada +2
We investigate the probability distribution of the effective inspiral spin, the mass ratio, and the chirp mass of primordial black hole (PBH) binaries, incorporating the effect of…
Dynamical photon sphere and time evolving shadow around black holes with temporal accretion
Yasutaka Koga, Nobuyuki Asaka, Masashi Kimura +1
A photon sphere is known as the geometrical structure shaping a black hole shadow. The mechanism is well understood for static or stationary black hole spacetimes such as the Schwa…
Photon surfaces in less symmetric spacetimes
Yasutaka Koga, Takahisa Igata, Keisuke Nakashi
We investigate photon surfaces and their stability in a less symmetric spacetime, a general static warped product with a warping function acting on a Riemannian submanifold of codi…
Spins of primordial black holes formed in the radiation-dominated phase of the universe: first-order effect
Tomohiro Harada, Chul-Moon Yoo, Kazunori Kohri +2
The standard deviation of the initial values of the nondimensional Kerr parameter of primordial black holes (PBHs) formed in the radiation-dominated phase of the universe i…
Sonic Point and Photon Surface
Masataka Tsuchiya, Chul-Moon Yoo, Yasutaka Koga +1
The sonic point/photon surface correspondence is thoroughly investigated in a general setting. First, we investigate a sonic point of a transonic steady perfect fluid flow in a gen…
Photon surfaces as pure tension shells: Uniqueness of thin shell wormholes
Yasutaka Koga
Thin shell wormholes are constructed by joining two asymptotically flat spacetimes along their inner boundaries. The junction conditions imposed on the spacetimes specify the equat…
Stability of null orbits on photon spheres and photon surfaces
Yasutaka Koga, Tomohiro Harada
Stability of a photon sphere, or stability of circular null geodesics on the sphere, plays a key role in its applications to astrophysics. For instance, an unstable photon sphere i…
Photon surfaces in spherically, planar and hyperbolically symmetric spacetimes of D-dimensions: Sonic point/photon sphere correspondence
Yasutaka Koga
Sonic point/photon sphere (SP/PS) correspondence is a theoretical phenomenon which appears in fluid dynamics on curved spacetime and its existence has been recently proved in quite…
Rotating accretion flows in dimensions: sonic points, critical points and photon spheres
Yasutaka Koga, Tomohiro Harada
We give the formulation and the general analysis of the rotational accretion problem on -dimensional spherical spacetime and investigate sonic points and critical points. First,…
The correspondence between sonic points of ideal photon gas accretion and photon spheres
Yasutaka Koga, Tomohiro Harada
In an accretion of fluid, its velocity may transit from subsonic to supersonic. The point at which such transition occurs is called sonic point and often mathematically special. We…
Cosmological long-wavelength solutions and primordial black hole formation
Tomohiro Harada, Chul-Moon Yoo, Tomohiro Nakama +1
We construct cosmological long-wavelength solutions without symmetry in general gauge conditions compatible with the long-wavelength scheme. We then specify the relationship among…