36 citations · 117 across the 4 of their papers we have counts for
8 papers
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…