Published Papers - Taniguchi Keisuke

Mass ejection from the merger of binary neutron stars

Hotokezaka Kenta, Kiuchi Kenta, Kyutoku Koutarou, Okawa Hirotada, Sekiguchi Yu-ichiro, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  87 ( 2 )   2013.01 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Numerical-relativity simulations for the merger of binary neutron stars are performed for a variety of equations of state (EOSs) and for a plausible range of the neutron-star mass, focusing primarily on the properties of the material ejected from the system. We find that a fraction of the material is ejected as a mildly relativistic and mildly anisotropic outflow with the typical and maximum velocities similar to 0.15-0.25c and similar to 0.5-0.8c (where c is the speed of light), respectively, and that the total ejected rest mass is in a wide range 10(-4)-10(-2)M(circle dot), which depends strongly on the EOS, the total mass, and the mass ratio. The total kinetic energy ejected is also in a wide range between 10(49) and 10(51) ergs. The numerical results suggest that for a binary of canonical total mass 2.7M(circle dot), the outflow could generate an electromagnetic signal observable by the planned telescopes through the production of heavy-element unstable nuclei via the r-process [6,20,21] or through the formation of blast waves during the interaction with the interstellar matter [7], if the EOS and mass of the binary are favorable ones. DOI: 10.1103/PhysRevD.87.024001

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Gravitational waves from black hole-neutron star binaries: Classification of waveforms (vol 79, 044030, 2009)

Shibata Masaru, Kyutoku Koutarou, Yamamoto Tetsuro, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  85 ( 12 )   2012.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Gravitational waves from spinning black hole-neutron star binaries: dependence on black hole spins and on neutron star equations of state

Kyutoku Koutarou, Okawa Hirotada, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  84 ( 6 )   2011.09 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We study the merger of black hole-neutron star binaries with a variety of black hole spins aligned or antialigned with the orbital angular momentum, and with the mass ratio in the range M-BH/M-NS = 2-5, where M-BH and M-NS are the mass of the black hole and neutron star, respectively. We model neutron-star matter by systematically parametrized piecewise polytropic equations of state. The initial condition is computed in the puncture framework adopting an isolated horizon framework to estimate the black hole spin and assuming an irrotational velocity field for the fluid inside the neutron star. Dynamical simulations are performed in full general relativity by an adaptive-mesh refinement code, SACRA. The treatment of hydrodynamic equations and estimation of the disk mass are improved. We find that the neutron star is tidally disrupted irrespective of the mass ratio when the black hole has a moderately large prograde spin, whereas only binaries with low mass ratios, M-BH/M-NS less than or similar to 3, or small compactnesses of the neutron stars bring the tidal disruption when the black hole spin is zero or retrograde. The mass of the remnant disk is accordingly large as greater than or similar to 0.1M(circle dot), which is required by central engines of short gamma-ray bursts, if the black hole spin is prograde. Information of the tidal disruption is reflected in a clear relation between the compactness of the neutron star and an appropriately defined "cutoff frequency" in the gravitational-wave spectrum, above which the spectrum damps exponentially. We find that the tidal disruption of the neutron star and excitation of the quasinormal mode of the remnant black hole occur in a compatible manner in high mass-ratio binaries with the prograde black hole spin. The correlation between the compactness and the cutoff frequency still holds for such cases. It is also suggested by extrapolation that the merger of an extremely spinning black hole and an irrotational neutron star binary does not lead to the formation of an overspinning black hole.

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New exact solutions of magnetic ellipsoidal figures of equilibrium

Kawamura Takumu, Taniguchi Keisuke, Yoshida Shin'ichirou, Eriguchi Yoshiharu

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY ( OXFORD UNIV PRESS )  416 ( 1 ) L75 - L79   2011.09 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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In this Letter we report brand new analytic stationary solutions of constant density stars with magnetic field and self-gravity. These solutions include prolate configurations even for purely poloidal magnetic fields as well as oblate configurations. These new analytic solutions are expressed in very simple forms and can be considered as generalized configurations of uniformly rotating constant density spheroids, i.e. Maclaurin spheroids, and of constant density ellipsoids with constant vorticity, i.e. Dedekind ellipsoids. As the axisymmetric Maclaurin spheroids and the triaxial Dedekind ellipsoids have been widely used for the estimation of the effect of rotations and/or internal motions on the self-gravitating bodies, our new analytic solutions may be used widely to estimate the effect of the magnetic fields semi-quantitatively in various contexts hereafter.

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The Japanese space gravitational wave antenna: DECIGO

Kawamura Seiji, Ando Masaki, Seto Naoki, Sato Shuichi, Nakamura Takashi, Tsubono Kimio, Kanda Nobuyuki, Tanaka Takahiro, Yokoyama Jun'ichi, Funaki Ikkoh, Numata Kenji, Ioka Kunihito, Takashima Takeshi, Agatsuma Kazuhiro, Akutsu Tomotada, Aoyanagi Koh-suke, Arai Koji, Araya Akito, Asada Hideki, Aso Yoichi, Chen Dan, Chiba Takeshi, Ebisuzaki Toshikazu, Ejiri Yumiko, Enoki Motohiro, Eriguchi Yoshiharu, Fujimoto Masa-Katsu, Fujita Ryuichi, Fukushima Mitsuhiro, Futamase Toshifumi, Harada Tomohiro, Hashimoto Tatsuaki, Hayama Kazuhiro, Hikida Wataru, Himemoto Yoshiaki, Hirabayashi Hisashi, Hiramatsu Takashi, Hong Feng-Lei, Horisawa Hideyuki, Hosokawa Mizuhiko, Ichiki Kiyotomo, Ikegami Takeshi, Inoue Kaiki T., Ishidoshiro Koji, Ishihara Hideki, Ishikawa Takehiko, Ishizaki Hideharu, Ito Hiroyuki, Itoh Yousuke, Izumi Kiwamu, Kawano Isao, Kawashima Nobuki, Kawazoe Fumiko, Kishimoto Naoko, Kiuchi Kenta, Kobayashi Shiho, Kohri Kazunori, Koizumi Hiroyuki, Kojima Yasufumi, Kokeyama Keiko, Kokuyama Wataru, Kotake Kei, Kozai Yoshihide, Kunimori Hiroo, Kuninaka Hitoshi, Kuroda Kazuaki, Kuroyanagi Sachiko, Maeda Kei-ichi, Matsuhara Hideo, Matsumoto Nobuyuki, Michimura Yuta, Miyakawa Osamu, Miyamoto Umpei, Miyoki Shinji, Morimoto Mutsuko Y., Morisawa Toshiyuki, Moriwaki Shigenori, Mukohyama Shinji, Musha Mitsuru, Nagano Shigeo, Naito Isao, Nakamura Kouji, Nakano Hiroyuki, Nakao Kenichi, Nakasuka Shinichi, Nakayama Yoshinori, Nakazawa Kazuhiro, Nishida Erina, Nishiyama Kazutaka, Nishizawa Atsushi, Niwa Yoshito, Noumi Taiga, Obuchi Yoshiyuki, Ohashi Masatake, Ohishi Naoko, Ohkawa Masashi, Okada Kenshi, Okada Norio, Oohara Kenichi, Sago Norichika, Saijo Motoyuki, Saito Ryo, Sakagami Masaaki, Sakai Shin-ichiro, Sakata Shihori, Sasaki Misao, Sato Takashi, Shibata Masaru, Shinkai Hisaaki, Shoda Ayaka, Somiya Kentaro, Sotani Hajime, Sugiyama Naoshi, Suwa Yudai, Suzuki Rieko, Tagoshi Hideyuki, Takahashi Fuminobu, Takahashi Kakeru, Takahashi Keitaro, Takahashi Ryutaro, Takahashi Ryuichi, Takahashi Tadayuki, Takahashi Hirotaka, Akiteru Takamori, Takano Tadashi, Tanaka Nobuyuki, Taniguchi Keisuke, Taruya Atsushi, Tashiro Hiroyuki, Torii Yasuo, Toyoshima Morio, Tsujikawa Shinji, Tsunesada Yoshiki, Ueda Akitoshi, Ueda Ken-ichi, Utashima Masayoshi, Wakabayashi Yaka, Yagi Kent, Yamakawa Hiroshi, Yamamoto Kazuhiro, Yamazaki Toshitaka, Yoo Chul-Moon, Yoshida Shijun, Yoshino Taizoh, Sun Ke-Xun

CLASSICAL AND QUANTUM GRAVITY ( IOP PUBLISHING LTD )  28 ( 9 )   2011.05 [ Peer Review Accepted ]

Type of publication: Research paper (international conference proceedings)

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The objectives of the DECi-hertz Interferometer Gravitational Wave Observatory (DECIGO) are to open a new window of observation for gravitational wave astronomy and to obtain insight into significant areas of science, such as verifying and characterizing inflation, determining the thermal history of the universe, characterizing dark energy, describing the formation mechanism of supermassive black holes in the center of galaxies, testing alternative theories of gravity, seeking black hole dark matter, understanding the physics of neutron stars and searching for planets around double neutron stars. DECIGO consists of four clusters of spacecraft in heliocentric orbits; each cluster employs three drag-free spacecraft, 1000 km apart from each other, whose relative displacements are measured by three pairs of differential Fabry-Perot Michelson interferometers. Two milestone missions, DECIGO pathfinder and Pre-DECIGO, will be launched to demonstrate required technologies and possibly to detect gravitational waves.

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Gravitational waves from nonspinning black hole-neutron star binaries: Dependence on equations of state

Kyutoku Koutarou, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  82 ( 4 )   2010.08 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We report results of a numerical-relativity simulation for the merger of a black hole-neutron star binary with a variety of equations of state (EOSs) modeled by piecewise polytropes. We focus, in particular, on the dependence of the gravitational waveform at the merger stage on the EOSs. The initial conditions are computed in the moving-puncture framework, assuming that the black hole is nonspinning and the neutron star has an irrotational velocity field. For a small mass ratio of the binaries (e.g., M-BH/M-NS = 2, where M-BH and M-NS are the masses of the black hole and neutron star, respectively), the neutron star is tidally disrupted before it is swallowed by the black hole irrespective of the EOS. Especially for less-compact neutron stars, the tidal disruption occurs at a more distant orbit. The tidal disruption is reflected in a cutoff frequency of the gravitational-wave spectrum, above which the spectrum amplitude exponentially decreases. A clear relation is found between the cutoff frequency of the gravitational-wave spectrum and the compactness of the neutron star. This relation also depends weakly on the stiffness of the EOS in the core region of the neutron star, suggesting that not only the compactness but also the EOS at high density is reflected in gravitational waveforms. The mass of the disk formed after the merger shows a similar correlation with the EOS, whereas the spin of the remnant black hole depends primarily on the mass ratio of the binary, and only weakly on the EOS. Properties of the remnant disks are also analyzed.

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BINARY NEUTRON STARS IN QUASI-EQUILIBRIUM

Taniguchi Keisuke, Shibata Masaru

ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES ( IOP PUBLISHING LTD )  188 ( 1 ) 187 - 208   2010.05 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Quasi-equilibrium sequences of binary neutron stars are constructed for a variety of equations of state in general relativity. Einstein's constraint equations in the Isenberg-Wilson-Mathews approximation are solved together with the relativistic equations of hydrostationary equilibrium under the assumption of irrotational flow. We focus on unequal-mass sequences as well as equal-mass sequences, and compare those results. We investigate the behavior of the binding energy and total angular momentum along a quasi-equilibrium sequence, the endpoint of sequences, and the orbital angular velocity as a function of time, changing the mass ratio, the total mass of the binary system, and the equation of state of a neutron star. It is found that the orbital angular velocity at the mass-shedding limit can be determined by an empirical formula derived from an analytic estimation. We also provide tables for 160 sequences, which will be useful as a guideline of numerical simulations for the inspiral and merger performed in the near future.

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DECIGO and DECIGO pathfinder

Ando Masaki, Kawamura Seiji, Seto Naoki, Sato Shuichi, Nakamura Takashi, Tsubono Kimio, Takashima Takeshi, Funaki Ikkoh, Numata Kenji, Kanda Nobuyuki, Tanaka Takahiro, Ioka Kunihito, Agatsuma Kazuhiro, Aoyanagi Koh-suke, Arai Koji, Araya Akito, Asada Hideki, Aso Yoichi, Chiba Takeshi, Ebisuzaki Toshikazu, Ejiri Yumiko, Enoki Motohiro, Eriguchi Yoshiharu, Fujimoto Masa-Katsu, Fujita Ryuichi, Fukushima Mitsuhiro, Futamase Toshifumi, Harada Tomohiro, Hashimoto Tatsuaki, Hayama Kazuhiro, Hikida Wataru, Himemoto Yoshiaki, Hirabayashi Hisashi, Hiramatsu Takashi, Hong Feng-Lei, Horisawa Hideyuki, Hosokawa Mizuhiko, Ichiki Kiyotomo, Ikegami Takeshi, Inoue Kaiki T., Ishidoshiro Koji, Ishihara Hideki, Ishikawa Takehiko, Ishizaki Hideharu, Ito Hiroyuki, Itoh Yousuke, Izumi Kiwamu, Kawano Isao, Kawashima Nobuki, Kawazoe Fumiko, Kishimoto Naoko, Kiuchi Kenta, Kobayashi Shiho, Kohri Kazunori, Koizumi Hiroyuki, Kojima Yasufumi, Kokeyama Keiko, Kokuyama Wataru, Kotake Kei, Kozai Yoshihide, Kunimori Hiroo, Kuninaka Hitoshi, Kuroda Kazuaki, Maeda Kei-ichi, Matsuhara Hideo, Mino Yasushi, Miyakawa Osamu, Miyamoto Umpei, Miyoki Shinji, Morimoto Mutsuko Y., Morisawa Toshiyuki, Moriwaki Shigenori, Mukohyama Shinji, Musha Mitsuru, Nagano Shigeo, Naito Isao, Nakamura Kouji, Nakamura Masahiro, Nakano Hiroyuki, Nakao Kenichi, Nakasuka Shinichi, Nakayama Yoshinori, Nakazawa Kazuhiro, Nishida Erina, Nishiyama Kazutaka, Nishizawa Atsushi, Niwa Yoshito, Noumi Taiga, Obuchi Yoshiyuki, Ohashi Masatake, Ohishi Naoko, Ohkawa Masashi, Okada Kenshi, Okada Norio, Oohara Kenichi, Sago Norichika, Saijo Motoyuki, Saito Ryo, Sakagami Masaaki, Sakai Shin-ichiro, Sakata Shihori, Sasaki Misao, Sato Takashi, Shibata Masaru, Shinkai Hisaaki, Somiya Kentaro, Sotani Hajime, Sugiyama Naoshi, Suwa Yudai, Suzuki Rieko, Tagoshi Hideyuki, Takahashi Fuminobu, Takahashi Kakeru, Takahashi Keitaro, Takahashi Ryutaro, Takahashi Ryuichi, Takahashi Tadayuki, Takahashi Hirotaka, Akiteru Takamori, Takano Tadashi, Taniguchi Keisuke, Taruya Atsushi, Tashiro Hiroyuki, Torii Yasuo, Toyoshima Morio, Tsujikawa Shinji, Tsunesada Yoshiki, Ueda Akitoshi, Ueda Ken-ichi, Utashima Masayoshi, Wakabayashi Yaka, Yagi Kent, Yamakawa Hiroshi, Yamamoto Kazuhiro, Yamazaki Toshitaka, Yokoyama Jun'ichi, Yoo Chul-Moon, Yoshida Shijun, Yoshino Taizoh, Sun Ken-Xun

CLASSICAL AND QUANTUM GRAVITY ( IOP PUBLISHING LTD )  27 ( 8 )   2010.04 [ Peer Review Accepted ]

Type of publication: Research paper (international conference proceedings)

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A space gravitational-wave antenna, DECIGO (DECI-hertz interferometer Gravitational wave Observatory), will provide fruitful insights into the universe, particularly on the formation mechanism of supermassive black holes, dark energy and the inflation of the universe. In the current pre-conceptual design, DECIGO will be comprising four interferometer units; each interferometer unit will be formed by three drag-free spacecraft with 1000 km separation. Since DECIGO will be an extremely challenging mission with high-precision formation flight with long baseline, it is important to increase the technical feasibility before its planned launch in 2027. Thus, we are planning to launch two milestone missions. DECIGO pathfinder (DPF) is the first milestone mission, and key components for DPF are being tested on ground and in orbit. In this paper, we review the conceptual design and current status of DECIGO and DPF.

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Exploring Binary-Neutron-Star-Merger Scenario of Short-Gamma-Ray Bursts by Gravitational-Wave Observation

Kiuchi Kenta, Sekiguchi Yuichiro, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW LETTERS ( AMER PHYSICAL SOC )  104 ( 14 ) 141101   2010.04 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We elucidate the feature of gravitational waves (GWs) from a binary-neutron-star merger collapsing to a black hole by general relativistic simulation. We show that GW spectrum imprints the coalescence dynamics, formation process of disk, equation of state for neutron stars, total masses, and mass ratio. A formation mechanism of the central engine of short-gamma-ray bursts, which are likely to be composed of a black hole and surrounding disk, therefore could be constrained by GW observation.

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Long-term general relativistic simulation of binary neutron stars collapsing to a black hole

Kiuchi Kenta, Sekiguchi Yuichiro, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  80 ( 6 )   2009.09 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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General relativistic simulations for the merger of binary neutron stars are performed as an extension of a previous work [M. Shibata and K. Taniguchi, Phys. Rev. D 73, 064027 (2006).]. We prepare binary neutron stars with a large initial orbital separation and employ the moving-puncture formulation, which enables one to follow merger and ringdown phases for a long time, even after black hole formation. For modeling inspiraling neutron stars, which should be composed of cold neutron stars, the Akmal-Pandharipande-Ravenhall (APR) equation of state (EOS) is adopted. After the onset of merger, the hybrid-type EOS is used; i.e., the cold and thermal parts are given by the APR and Gamma-law EOSs, respectively. Three equal-mass binaries, each with mass 1:4M(circle dot), 1.45M(circle dot), and 1.5M(circle dot), and two unequal-mass binaries with mass, 1.3 vs 1.6M(circle dot) and 1.35 vs 1.65M(circle dot), are prepared. We focus primarily on the black hole formation case, and explore mass and spin of the black hole, mass of disks which surround the black hole, and gravitational waves emitted during the black hole formation. We find that (i) the black hole is promptly formed if total mass of the system initially satisfies m(0) >= 2.9M(circle dot); (ii) for the systems of m(0) = 2.9-3.0M(circle dot) and of mass ratio approximate to 0.8, the mass of disks which surround the formed black hole is 0.006-0.02M(circle dot); (iii) the spin of the formed black hole is 0.78 +/- 0.02 when a black hole is formed after the merger in the dynamical time scale. This value depends weakly on the total mass and mass ratio, and is about 0.1 larger than that of a black hole formed from nonspinning binary black holes; (iv) for the black hole formation case, Fourier spectrum shape of gravitational waves emitted in the merger and ringdown phases has a universal qualitative feature irrespective of the total mass and mass ratio, but quantitatively, the spectrum reflects the parameters of the binary neutron stars.

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Quasiequilibrium states of black hole-neutron star binaries in the moving-puncture framework

Kyutoku Koutarou, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  79 ( 12 )   2009.06 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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General relativistic quasiequilibrium states of black hole-neutron star binaries are computed in the moving-puncture framework. We propose three conditions for determining the quasiequilibrium states and compare the numerical results with those obtained in the excision framework. We find that the results obtained in the moving-puncture framework agree with those in the excision framework and with those in the third post-Newtonian approximation for the cases that (i) the mass ratio of the binary is close to unity irrespective of the orbital separation, and (ii) the orbital separation is large enough (m(0)Omega less than or similar to 0.02, where m(0) and Omega are the total mass and the orbital angular velocity, respectively) irrespective of the mass ratio. For m(0)Omega greater than or similar to 0.03, both of the results in the moving-puncture and excision frameworks deviate, more or less, from those in the third post-Newtonian approximation. Thus the numerical results do not provide a quasicircular state, rather they seem to have a non-negligible eccentricity of order 0.01-0.1. We show by numerical simulation that a method in the moving-puncture framework can provide approximately quasicircular states in which the eccentricity is by a factor of similar to 2 smaller than those in quasiequilibrium given by other approaches.

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DECIGO pathfinder

Ando Masaki, Kawamura Seiji, Sato Shuichi, Nakamura Takashi, Tsubono Kimio, Araya Akito, Funaki Ikkoh, Ioka Kunihito, Kanda Nobuyuki, Moriwaki Shigenori, Musha Mitsuru, Nakazawa Kazuhiro, Numata Kenji, Sakai Shin-ichiro, Seto Naoki, Takashima Takeshi, Tanaka Takahiro, Agatsuma Kazuhiro, Aoyanagi Koh-suke, Arai Koji, Asada Hideki, Aso Yoichi, Chiba Takeshi, Ebisuzaki Toshikazu, Ejiri Yumiko, Enoki Motohiro, Eriguchi Yoshiharu, Fujimoto Masa-Katsu, Fujita Ryuichi, Fukushima Mitsuhiro, Futamase Toshifumi, Ganzu Katsuhiko, Harada Tomohiro, Hashimoto Tatsuaki, Hayama Kazuhiro, Hikida Wataru, Himemoto Yoshiaki, Hirabayashi Hisashi, Hiramatsu Takashi, Hong Feng-Lei, Horisawa Hideyuki, Hosokawa Mizuhiko, Ichiki Kiyotomo, Ikegami Takeshi, Inoue Kaiki T., Ishidoshiro Koji, Ishihara Hideki, Ishikawa Takehiko, Ishizaki Hideharu, Ito Hiroyuki, Itoh Yousuke, Kawashima Nobuki, Kawazoe Fumiko, Kishimoto Naoko, Kiuchi Kenta, Kobayashi Shiho, Kohri Kazunori, Koizumi Hiroyuki, Kojima Yasufumi, Kokeyama Keiko, Kokuyama Wataru, Kotake Kei, Kozai Yoshihide, Kudoh Hideaki, Kunimori Hiroo, Kuninaka Hitoshi, Kuroda Kazuaki, Maeda Kei-ichi, Matsuhara Hideo, Mino Yasushi, Miyakawa Osamu, Miyoki Shinji, Morimoto Mutsuko Y., Morioka Tomoko, Morisawa Toshiyuki, Mukohyama Shinji, Nagano Shigeo, Naito Isao, Nakamura Kouji, Nakano Hiroyuki, Nakao Kenichi, Nakasuka Shinichi, Nakayama Yoshinori, Nishida Erina, Nishiyama Kazutaka, Nishizawa Atsushi, Niwa Yoshito, Noumi Taiga, Obuchi Yoshiyuki, Ohashi Masatake, Ohishi Naoko, Ohkawa Masashi, Okada Norio, Onozato Kouji, Oohara Kenichi, Sago Norichika, Saijo Motoyuki, Sakagami Masaaki, Sakata Shihori, Sasaki Misao, Sato Takashi, Shibata Masaru, Shinkai Hisaaki, Somiya Kentaro, Sotani Hajime, Sugiyama Naoshi, Suwa Yudai, Suzuki Rieko, Tagoshi Hideyuki, Takahashi Fuminobu, Takahashi Kakeru, Takahashi Keitaro, Takahashi Ryutaro, Takahashi Ryuichi, Takahashi Tadayuki, Takahashi Hirotaka, Akiteru Takamori, Takano Tadashi, Taniguchi Keisuke, Taruya Atsushi, Tashiro Hiroyuki, Torii Yasuo, Toyoshima Morio, Tsujikawa Shinji, Tsunesada Yoshiki, Ueda Akitoshi, Ueda Ken-ichi, Utashima Masayoshi, Wakabayashi Yaka, Yamakawa Hiroshi, Yamamoto Kazuhiro, Yamazaki Toshitaka, Yokoyama Jun'ichi, Yoo Chul-Moon, Yoshida Shijun, Yoshino Taizoh

CLASSICAL AND QUANTUM GRAVITY ( IOP PUBLISHING LTD )  26 ( 9 )   2009.05 [ Peer Review Accepted ]

Type of publication: Research paper (international conference proceedings)

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DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory), which is a future space gravitational wave antenna. DECIGO is expected to provide fruitful insights into the universe, particularly about dark energy, the formation mechanism of supermassive black holes and the inflation of the universe. Since DECIGO will be an extremely challenging mission, which will be formed by three drag-free spacecraft with 1000 km separation, it is important to increase the technical feasibility of DECIGO before its planned launch in 2024. Thus, we are planning to launch two milestone missions: DPF and pre-DECIGO. In this paper, we review the conceptual design and current status of the first milestone mission, DPF.

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Equilibrium initial data for moving puncture simulations: the stationary 1+log slicing

Baumgarte T. W., Etienne Z. B., Liu Y. T., Matera K., Murchadha N. O., Shapiro S. L., Taniguchi K.

CLASSICAL AND QUANTUM GRAVITY ( IOP Publishing )  26 ( 8 ) 085007 - 085007   2009.04 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Gravitational waves from black hole-neutron star binaries: Classification of waveforms

Shibata Masaru, Kyutoku Koutarou, Yamamoto Tetsuro, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  79 ( 4 )   2009.02 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Using our new numerical-relativity code SACRA, long-term simulations for inspiral and merger of black hole (BH)-neutron star (NS) binaries are performed, focusing particularly on gravitational waveforms. As the initial conditions, BH-NS binaries in a quasiequilibrium state are prepared in a modified version of the moving-puncture approach. The BH is modeled by a nonspinning moving puncture and, for the NS, a polytropic equation of state with Gamma=2 and the irrotational velocity field are employed. The mass ratio of the BH to the NS, Q=M-BH/M-NS, is chosen in the range between 1.5 and 5. The compactness of the NS, defined by C=GM(NS)/c(2)R(NS), is chosen to be between 0.145 and 0.178. For a large value of Q for which the NS is not tidally disrupted and is simply swallowed by the BH, gravitational waves are characterized by inspiral, merger and ringdown waveforms. In this case, the waveforms are qualitatively the same as that from BH-BH binaries. For a sufficiently small value of Q less than or similar to 2, the NS may be tidally disrupted before it is swallowed by the BH. In this case, the amplitude of the merger and ringdown waveforms is very low, and thus, gravitational waves are characterized by the inspiral waveform and subsequent quick damping. The difference in the merger and ringdown waveforms is clearly reflected in the spectrum shape and in the "cutoff" frequency above which the spectrum amplitude steeply decreases. When a NS is not tidally disrupted (e.g., for Q=5), kick velocity, induced by asymmetric gravitational-wave emission, agrees approximately with that derived for the merger of BH-BH binaries, whereas for the case when the tidal disruption occurs, the kick velocity is significantly suppressed.

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Simulating coalescing compact binaries by a new code (SACRA)

Yamamoto Tetsuro, Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  78 ( 6 )   2008.09 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We report our new code, named SACRA (SimulAtor for Compact objects in Relativistic Astrophysics) for numerical relativity simulations in which an adaptive mesh refinement algorithm is implemented. In this code, the Einstein equations are solved in the Baumgarte-Shapiro-Shibata-Nakamura formalism with a fourth-order finite differencing, and the hydrodynamic equations are solved by a third-order high-resolution central scheme. The fourth-order Runge-Kutta scheme is adopted for integration in time. To test the code, simulations for coalescence of black hole-black hole, neutron star-neutron star (NS-NS), and black hole-neutron star (BH-NS) binaries are performed, and also, properties of BHs formed after the merger and gravitational waveforms are compared among those three cases. For the simulations of black hole-black hole binaries, we adopt the same initial conditions as those by Buonanno et al. [1] and compare numerical results. We find reasonable agreement except for a slight disagreement possibly associated with the difference in choice of gauge conditions and numerical schemes. For an NS-NS binary, we performed simulations employing both SACRA and Shibata's previous code, and find reasonable agreement between two numerical results for the final outcome and qualitative property of gravitational waveforms. We also find that the convergence is relatively slow for numerical results of NS-NS binaries, and again realize that long-term numerical simulations with several resolutions and grid settings are required for validating the results. For a BH-NS binary, we compare numerical results with our previous ones, and find that gravitational waveforms and properties of the BH formed after the merger agree well with those of our previous ones, although the disk mass formed after the merger is less than 0.1% of the total rest mass, which disagrees with the previous result. We also report numerical results of a long-term simulation (with similar to 4 orbits) for a BH-NS binary for the first time. All these numerical results show behavior of convergence, and extrapolated numerical results for time spent in the inspiral phase agree with post-Newtonian predictions in a reasonable accuracy. These facts validate the results by SACRA.

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General relativistic simulations of magnetized binary neutron star mergers

Liu Yuk Tung, Shapiro Stuart L., Etienne Zachariah B., Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  78 ( 2 )   2008.07 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Binary neutron stars (NSNS) are expected to be among the leading sources of gravitational waves observable by ground-based laser interferometers and may be the progenitors of short-hard gamma-ray bursts. We present a series of general relativistic NSNS coalescence simulations both for unmagnetized and magnetized stars. We adopt quasiequilibrium initial data for circular, irrotational binaries constructed in the conformal thin-sandwich (CTS) framework. We adopt the BSSN formulation for evolving the metric and a high-resolution shock-capturing scheme to handle the magnetohydrodynamics. Our simulations of unmagnetized binaries agree with the results of Shibata, Taniguchi and Uryu [M. Shibata, K. Taniguchi, and K. Uryu, Phys. Rev. D 68, 084020 (2003).]. In cases in which the mergers result in a prompt collapse to a black hole, we are able to use puncture gauge conditions to extend the evolution and determine the mass of the material that forms a disk. We find that the disk mass is less than 2% of the total mass in all cases studied. We then add a small poloidal magnetic field to the initial configurations and study the subsequent evolution. For cases in which the remnant is a hypermassive neutron star, we see measurable differences in both the amplitude and phase of the gravitational waveforms following the merger. For cases in which the remnant is a black hole surrounded by a disk, the disk mass and the gravitational waveforms are about the same as the unmagnetized cases. Magnetic fields substantially affect the long-term, secular evolution of a hypermassive neutron star (driving "delayed collapse") and an accretion disk around a nascent black hole.

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Fully general relativistic simulations of black hole-neutron star mergers

Etienne Zachariah B., Faber Joshua A., Liu Yuk Tung, Shapiro Stuart L., Taniguchi Keisuke, Baumgarte Thomas W.

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  77 ( 8 )   2008.04 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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Black hole-neutron star (BHNS) binaries are expected to be among the leading sources of gravitational waves observable by ground-based detectors, and may be the progenitors of short-hard gamma-ray bursts (SGRBs) as well. We discuss our new fully general relativistic calculations of merging BHNS binaries, which use high-accuracy, low-eccentricity, conformal thin-sandwich configurations as initial data. Our evolutions are performed using the moving puncture method and include a fully relativistic, high-resolution shock-capturing hydrodynamics treatment. Focusing on systems in which the neutron star is irrotational and the black hole is nonspinning with a 31 mass ratio, we investigate the inspiral, merger, and disk formation in the system. We find that the vast majority of material is promptly accreted and no more than 3% of the neutron star's rest mass is ejected into a tenuous, gravitationally bound disk. We find similar results for mass ratios of 21 and 11, even when we reduce the neutron stars (NS) compaction in the 21 mass ratio case. These ambient disks reach temperatures suitable for triggering SGRBs, but their masses may be too small to produce the required total energy output. We measure gravitational waveforms and compute the effective strain in frequency space, finding measurable differences between our waveforms and those produced by binary black hole mergers within the advanced LIGO band. These differences appear at frequencies corresponding to the emission that occurs when the NS is tidally disrupted and accreted by the black hole. The resulting information about the radius of the neutron star may be used to constrain the neutron star equation of state.

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Merger of black hole and neutron star in general relativity: Tidal disruption, torus mass, and gravitational waves

Shibata Masaru, Taniguchi Keisuke

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  77 ( 8 )   2008.04 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We study the properties of the merger of black hole-neutron star (BH-NS) binaries in fully general relativistic simulation, focusing on the case that the NS is tidally disrupted. We prepare BH-NS binaries in a quasicircular orbit as the initial condition in which the BH is modeled by a nonspinning moving puncture. For modeling the NS, we adopt the Gamma-law equation of state with Gamma=2 and the irrotational velocity field. We change the BH mass in the range M-BH approximate to 3.3M-4.6M, while the rest mass of the NS is fixed to be M-* = 1.4M (i.e., the NS mass M-NS approximate to 1.3M). The radius of the corresponding spherical NS is set in the range R-NS approximate to 12-15 km (i.e., the compactness GM(NS)/R(NS)c(2) approximate to 0.13-0.16). We find for all the chosen initial conditions that the NS is tidally disrupted near the innermost stable circular orbit. The numerical results indicate that, for the model of R-NS = 12 km, more than similar to 95% of the rest mass is quickly swallowed by the BH and the resultant torus mass surrounding the BH is less than similar to 0.05M. For the model of R-NS approximate to 14.7 km, by contrast, the torus mass is similar to 0.15M for the BH mass approximate to 4M. The thermal energy of the material in the torus increases due to the shock heating that occurs in the collision between the spiral arms, resulting in the temperature 10(10)-10(11) K. Our results indicate that the merger between a low-mass BH and its companion NS may form a central engine of short gamma-ray bursts of the total energy of order 10(49) ergs if the compactness of the NS is fairly small, less than or similar to 0.145. However, for the canonical values M-NS = 1.35M and R-NS = 12 km, the merger results in small torus mass, and hence it can be a candidate only for the low-energy, short gamma-ray bursts of total energy of order 10(48) ergs. We also present gravitational waveforms during the inspiral, tidal disruption of the NS, and subsequent evolution of the disrupted material. We find that the amplitude of gravitational waves quickly decreases after the onset of tidal disruption. Although the quasinormal mode is excited, its gravitational wave amplitude is much smaller than that of the late inspiral phase. This is reflected in the fact that the spectrum amplitude sharply falls above a cutoff frequency which is determined by the tidal disruption process. We also find that the cutoff frequency is 1.25-1.4 times larger than the frequency predicted by the study for the sequence of the quasicircular orbits, and this factor of the deviation depends on the compactness of the NS.

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Relativistic black hole-neutron star binaries in quasiequilibrium: Effects of the black hole excision boundary condition

Taniguchi Keisuke, Baumgarte Thomas W., Faber Joshua A., Shapiro Stuart L.

PHYSICAL REVIEW D ( AMER PHYSICAL SOC )  77 ( 4 )   2008.02 [ Peer Review Accepted ]

Type of publication: Research paper (scientific journal)

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We construct new models of black hole-neutron star binaries in quasiequilibrium circular orbits by solving Einstein's constraint equations in the conformal thin-sandwich decomposition together with the relativistic equations of hydrostationary equilibrium. We adopt maximal slicing, assume spatial conformal flatness, and impose equilibrium boundary conditions on an excision surface (i.e., the apparent horizon) to model the black hole. In our previous treatment we adopted a "leading-order" approximation for a parameter related to the black hole spin in these boundary conditions to construct approximately nonspinning black holes. Here we improve on the models by computing the black hole's quasilocal spin angular momentum and setting it to zero. As before, we adopt a polytropic equation of state with adiabatic index Gamma=2 and assume the neutron star to be irrotational. In addition to recomputing several sequences for comparison with our earlier results, we study a wider range of neutron star masses and binary mass ratios. To locate the innermost stable circular orbit we search for turning points along both the binding energy and total angular momentum curves for these sequences. Unlike for our previous approximate boundary condition, these two minima now coincide. We also identify the formation of cusps on the neutron star surface, indicating the onset of tidal disruption. Comparing these two critical binary separations for different mass ratios and neutron star compactions we distinguish those regions that will lead to a tidal disruption of the neutron star from those that will result in the plunge into the black hole of a neutron star more or less intact, albeit distorted by tidal forces.

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DECIGO Pathfinder

Masaki Ando, Seiji Kawamura, Takashi Nakamura, Kimio Tsubono, Takahiro Tanaka, Ikkoh Funaki, Naoki Seto, Kenji Numata, Shuichia Sato, Kunihito Ioka, Nobuyuki Kanda, Takeshi Takashima, Kazuhiro Agatsuma, Tomotada Akutsu, Tomomi Akutsu, Koh-suke Aoyanagi, Koji Arai, Yuta Arase, Akito Araya, Hideki Asada, Yoichi Aso, Takeshi Chiba, Toshikazu Ebisuzaki, Motohiro Enoki, Yoshiharu Eriguchi, Masa-Katsu Fujimoto, Ryuichi Fujita, Mitsuhiro Fukushima, Toshifumi Futamase, Katsuhiko Ganzu, Tomohiro Harada, Tatsuaki Hashimoto, Kazuhiro Hayama, Wataru Hikida, Yoshiaki Himemoto, Hisashi Hirabayashi, Takashi Hiramatsu, Feng-Lei Hong, Hideyuki Horisawa, Mizuhiko Hosokawa, Kiyotomo Ichiki, Takeshi Ikegami, Kaiki T. Inoue, Koji Ishidoshiro, Hideki Ishihara, Takehiko Ishikawa, Hideharu Ishizaki, Hiroyuki Ito, Yousuke Itoh, Shogo Kamagasako, Nobuki Kawashima, Fumiko Kawazoe, Hiroyuki Kirihara, Naoko Kishimoto, Kenta Kiuche, Shiho Kobayashi, Kazunori Kohri, Hiroyuki Koizumi, Yasufumi Kojima, Keiko Kokeyama, Wataru Kokuyama, Kei Kotake, Yoshihide Kozai, Hideaki Kudoh, Hiroo Kunimori, Hitoshi Kuninaka, Kazuaki Kuroda, Kei-ichi Maeda, Hideo Matsuhara, Yasushi Mino, Osamu Miyakawa, Shinji Miyoki, Mutsuko Y. Morimoto, Tomoko Morioka, Toshiyuki Morisawa, Shigenori Moriwaki, Shinji Mukohyama, Mitsuru Musha, Shigeo Nagano, Isao Naito, Noriyasu Nakagawa, Kouji Nakamura, Hiroyuki Nakano, Kenichi Nakao, Shinichi Nakasuka, Yoshinori Nakayama, Erina Nishida, Kazutaka Nishiyama, Atsushi Nishizawa, Yoshito Niwa, Masatake Ohashi, Naoko Ohishi, Masashi Ohkawa, Akira Okutomi, Kouji Onozato, Kenichi Oohara, Norichika Sago, Motoyuki Saijo, Masaaki Sakagami, Shin-ichiro Sakai, Shihori Sakata, Misao Sasaki, Takashi Sato, Masaru Shibata, Hisaaki Shinkai, Kentaro Somiya, Hajime Sotani, Naoshi Sugiyama, Yudai Suwa, Hideyuki Tagoshi, Kakeru Takahashi, Keitaro Takahashi, Tadayuki Takahashi, Hirotaka Takahashi, Ryuichi Takahashi, Ryutaro Takahashi, Akiteru Takamori, Tadashi Takano, Keisuke Taniguchi, Atsushi Taruya, Hiroyuki Tashiro, Mitsuru Tokuda, Masao Tokunari, Morio Toyoshima, Shinji Tsujikawa, Yoshiki Tsunesada, Ken-ichi Ueda, Masayoshi Utashima, Hiroshi Yamakawa, Kazuhiro Yamamoto, Toshitaka Yamazaki, Jun'ichi Yokoyama, Chul-Moon Yoo, Shijun Yoshida, Taizoh Yoshino

TAUP2007: TENTH INTERNATIONAL CONFERENCE ON TOPICS IN ASTROPARTICLE AND UNDERGROUND PHYSICS ( IOP PUBLISHING LTD )  120   2008 [ Peer Review Accepted ]

Type of publication: Research paper (international conference proceedings)

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DECIGO pathfinder (DPF) is a milestone satellite mission for DECIGO (DECi-hertz Interferometer Gravitational wave Observatory) which is a future space gravitational wave antenna. DECIGO is expected to provide us fruitful insights into the universe, in particular about dark energy, a fori-nation mechanism of supermassive black holes, and the inflation of the universe. Since DECIGO will be an extremely large mission which will formed by three drag-free spacecraft with 1000m separation, it is significant to gain the technical feasibility of DECIGO before its planned launch in 2024. Thus, we are planning to launch two milestone missions: DPF and pre-DECIGO. The conceptual design and current status of the first milestone mission, DPF, are reviewed in this article.

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