Curriculum Vitaes

Takahisa Igata

  (伊形 尚久)

Profile Information

Affiliation
Assistant Professor, Faculty of Science Department of Physics, Gakushuin University
Degree
Doctor (Science)(Mar, 2012, Graduate School of Science, Osaka City University )

Researcher number
40711487
ORCID ID
 https://orcid.org/0000-0002-3344-9045
J-GLOBAL ID
201801018312811299
researchmap Member ID
B000325603

Papers

 45
  • Takahisa Igata, Tomohiro Harada, Hiromi Saida, Yohsuke Takamori
    International Journal of Modern Physics D, 32(16) 2350105, Dec, 2023  Peer-reviewed
    We consider the periapsis shifts of bound orbits of stars on static clouds around a black hole. The background spacetime is constructed from a Schwarzschild black hole surrounded by a static and spherically symmetric self-gravitating system of massive particles, which satisfies all the standard energy conditions and physically models the gravitational effect of dark matter distribution around a nonrotating black hole. Using nearly circular bound orbits of stars, we obtain a simple formula for the precession rate. This formula explicitly shows that the precession rate is determined by a positive contribution (i.e. a prograde shift) from the conventional general-relativistic effect and a negative contribution (i.e. a retrograde shift) from the local matter density. The four quantities for such an orbit (i.e. the orbital shift angle, the radial oscillation period, the redshift and the star position mapped onto the celestial sphere) determine the local values of the background model functions. Furthermore, we not only evaluate the precession rate of nearly circular bound orbits in several specific models but also simulate several bound orbits with large eccentricity and their periapsis shifts. The present exact model demonstrates that the retrograde precession does not mean any exotic central objects such as naked singularities or wormholes but simply the existence of significant energy density of matters on the star orbit around the black hole.
  • Tomohiro Harada, Takahisa Igata, Hiromi Saida, Yohsuke Takamori
    International Journal of Modern Physics D, 32(15) 2350098, Nov, 2023  Peer-reviewed
  • Tomohiro Harada, Takahisa Igata, Sato, Takuma, Carr, Bernard
    Classical and Quantum Gravity, 39(14) 145008, Jun 29, 2022  Peer-reviewed
    We completely classify the Friedmann–Lemaître–Robertson–Walker solutions with spatial curvature K = 0, ±1 for perfect fluids with linear equation of state p = wρ, where ρ and p are the energy density and pressure, without assuming any energy conditions. We extend our previous work to include all geodesics and parallelly propagated (p.p.) curvature singularities, showing that no non-null geodesic emanates from or terminates at the null portion of conformal infinity and that the initial singularity for K = 0, −1 and −5/3 < w < −1 is a null non-scalar polynomial curvature singularity. We thus obtain the Penrose diagrams for all possible cases and identify w = −5/3 as a critical value for both the future big-rip singularity and the past null conformal boundary.
  • Takahisa Igata, Yohsuke Takamori
    Physical Review D, 105(12) 124029, Jun 15, 2022  Peer-reviewed
  • Kota Ogasawara, Takahisa Igata
    Physical Review D, 105(2) 024031, Jan 10, 2022  Peer-reviewed
    We consider necessary and sufficient conditions for photons emitted from an arbitrary spacetime position of the extremal Kerr black hole to escape to infinity. The radial equation of motion determines necessary conditions for photons emitted from r=r∗ to escape to infinity, and the polar angle equation of motion further restricts the allowed region of photon motion. From these two conditions, we provide a method to visualize a two-dimensional photon impact parameter space that allows photons to escape to infinity, i.e., the escapable region. Finally, we completely identify the escapable region for the extremal Kerr black hole spacetime. This study has generalized our previous result [K.~Ogasawara and T.~Igata, Phys. Rev. D \textbf{103}, 044029 (2021)], which focused only on light sources near the horizon, to the classification covering light sources in the entire region.

Misc.

 3
  • Shunichiro Kinoshita, Takahisa Igata
    74(8) 542-547, Aug, 2019  Peer-reviewedInvited
    The Blandford‒Znajek process is an energy-extraction mech- anism from a rotating black hole by force-free electromagnetic fields. Since this process can efficiently achieve powerful energy fluxes, it has been widely believed to be a viable mechanism for the formation of relativistic jets. From a spacetime perspective, the dynamics of magnet- ic field lines of force-free electromagnetic fields can be rewritten into a quite similar form for the dynamics of strings. Using this formalism, we explicitly show that the energy and angular-momentum fluxes for stationary and axisymmetric force-free electromagnetic fields have identical properties to those for rigidly rotating Nambu‒Goto strings. Thus, we conclude that the Blandford‒Znajek process is kinematically identical to an energy-extraction mechanism by the Nambu‒Goto string and the magnetic field lines with magnetic tension play an important role in the Blandford‒Znajek process.
  • Takahisa Igata, Hideki Ishihara, Yohsuke Takamori
    The Thirteenth Marcel Grossmann Meeting, Mar, 2015  Peer-reviewed
    We show that there exist chaotic bound orbits of a particle around a singly rotating black ring in five-dimensions by using Poincaré map.
  • Takahisa Igata, Hideki Ishihara, Yohsuke Takamori
    Journal of Physics: Conference Series, 314 012113-012113, Sep 22, 2011  Peer-reviewed
    We study stable bound orbits of a free particle around a black ring. Unlike the higher-dimensional black hole case, we find that there exist stable bound orbits in toroidal spiral shape near the ring axis and stable circular orbits on the axis. In addition, radii of stable bound orbits can be infinitely large if the ring thickness is less than a critical value.

Teaching Experience

 13

Professional Memberships

 6

Research Projects

 6