Citation of the 2018 Nishina Memorial Prize1) Awardee:
Dr. Masaru Shibata
Director, Max Planck Institute for Gravitational Physics
Professor, Yukawa Institute for Theoretical Physics,
Kyoto University

Title of Research Achievement“Study of Binary Neutron Star Mergers by Numerical
Relativity Simulations”

Abstract of Research Achievement
Mergers of close binary neutron stars are important events not just to verify
general relativity involving very strong gravitational fields, but also to provide
information on the internal structure of neutron stars and the astrophysical originof heavy elements.
Devoted to theoretical research on relativistic merger events for more than 20years, Dr. Shibata has established the canonical scenario of the merger, and has
characterized model predictions of various observables to be seen in the merger
events. His research started with extensive studies on numerical relativity where theEinstein equation and the relativistic hydrodynamics equations are solved
simultaneously. He developed a wide array of approaches such as the choice ofrelevant gauge conditions, establishment of stable numerical hydrodynamics codes,
the numerical method to define the gravitational collapse into a black hole, and themethod to extract gravitational-wave signals. In 1999, Dr. Shibata successfully
performed the world's first numerical simulations of merging binary neutron stars
by integrating those approaches [1].
This work was highly praised by the community for ushering in the practicalapplication of numerical relativity simulations. Later, as the lead of his team of young
researchers, Dr. Shibata enhanced the precision of physical processes as well as thecomputational resolution, and performed a series of simulations of various merger
models under realistic conditions [2,3,4,5]; he founded standard theories on
dynamical merger processes, gravitational waveforms, mass ejection and neutrino
emission, among others, and progressed with theoretical predictions of the behaviorof gravitational waves and electromagnetic radiation in binary neutron star mergers.

Coinciding with his scientific achievement, the gravitational wave detectors,
Advanced LIGO and Advanced Virgo, recorded for the first time the gravitational
waves (GW170817) from a merger of binary neutron stars in August 2017.
Furthermore, simultaneous observation of electromagnetic waves, ranging fromgamma rays to infrared radiation, marked the herald of "multi-messengerastronomy". Dr. Shibata’s theoretical research has become a fundamental key toelucidating physical phenomena behind these observations [6].
Dr. Shibata is certainly the pioneer and the front runner in the research ofbinary neutron star mergers based on numerical relativity simulations. Coupledwith expected further advancements in observations of gravitational waves andelectromagnetic counterparts, Dr. Shibata’s work will undoubtedly gain increasing
importance.

[1] “Fully general relativistic simulation of coalescing binary neutron stars:
Preparatory test”, M. Shibata, Phys. Rev. D 60, 104052-1-25 (1999).
[2] “Simulation of merging binary neutron stars in full general relativity: Γ=2 case”,
M. Shibata and K. Uryu, Phys. Rev. D 61, 064001-1-18 (2000).
[3] “Merger of binary neutron stars with realistic equations of state in full generalrelativity”, M. Shibata, K. Taniguchi, and K. Uryu, Phys. Rev. D 71, 084021-1-26(2005).
[4] “The mass ejection from the merger of binary neutron stars”, K. Hotokezaka, K.
Kiuchi, K. Kyutoku, H. Okawa, Y. Sekiguchi, M. Shibata, and K. Taniguchi,Phys. Rev. D 87, 024001-1-27 (2013).
[5] “Remnant massive neutron stars of binary neutron star mergers: Evolution
process and gravitational waveform”, K. Hotokezaka, K. Kiuchi, K. Kyutoku, T.
Muranushi, Y. Sekiguchi, M. Shibata, and K. Taniguchi, Phys. Rev. D 88,
044026-1-30 (2013).
[6] “Modeling GW170817 based on numerical relativity and its implications”, M.
Shibata, S. Fujibayashi, K. Hotokezaka, K. Kiuchi, K. Kyutoku, Y. Sekiguchi,and M. Tanaka, Phys. Rev. D 96, 123012-1-22 (2017).

2) Awardee:
Dr. Koichiro Tanaka
Professor, Graduate School of Science, Kyoto University

Title of Research Achievement“Development of Extreme-nonlinear Terahertz Optics inSolids”

Abstract of Research Achievement
Extreme-nonlinear optical phenomena are induced
when the interaction energy of an electron system in matter with incident light getsclose to or exceeds the characteristic energy scale of the electron system (such asionization or band-gap energy). In the 1990's it was experimentally discovered thathigher-order harmonic light waves are generated when atoms are irradiated withhigh-intensity visible light pulses. For the description of such optical phenomena,
perturbation theory is no longer appropriate, and the development of a novel
theoretical framework, including the case when matter and light highly interact witheach other, was strongly called for. Subsequently, the proposal of a successful
phenomenological model triggered the theoretical understanding. At the same time,
technologies for generating soft X rays from visible and attosecond light sources weredeveloped by employing extreme-nonlinear optical effects. On the other hand, a solid
is damaged when it is irradiated with high-intensity visible light pulses such as thoseused for atoms, so that a profound understanding of extreme-nonlinear opticalphenomena in solids had not been achieved yet. In solids consisting of a periodic
crystal lattice, the electron system is well described by the band picture, but to what
extent this band picture is applicable when the intensity of incoming light is causing
extreme-nonlinear optical phenomena and also what kind of optical phenomena
appear, remained unknown.
Dr. Tanaka has successfully developed a high-intensity terahertz light source
based on the pulse-front-tilting method, according to his idea that extreme-nonlinear
optical phenomena in solids can be observed without damaging them by reducing thelight frequency to the terahertz region. As a result, he has realized the world's firstterahertz light source where the pulse peak electric field exceeds 1.2 MV/cm (peakmagnetic field is 0.4 T) [1]. This has triggered studies on terahertz extreme-nonlinear
optics in solids all over the world with great momentum in both experiment andtheory, leading to a sheer endless series of discoveries. Dr. Tanaka himself used thislight source to realize a 1000-times amplification of charge carriers in GaAs quantum
wells [2], a giant modulation of the magnetization in the canted ferromagnet HoFeO3[3], ultrafast terahertz optical switching [4], and the generation of higher-order highharmonic light in graphene for the first time in the world [5].
.
[1] "Single-cycle terahertz pulses with amplitudes exceeding 1 MV/cm generated by
optical rectification in LiNbO3", H. Hirori, A. Doi, F. Blanchard, and K. Tanaka,
Appl. Phys. Lett. 98, 091106 (2011).
[2] "Extraordinary Carrier Multiplication Gated by a Picosecond Electric Field
Pulse", H. Hirori, K. Shinokita, M.Shirai, S.Tani, Y. Kadoya, and K. Tanaka,
Nature Communications 2, 594 (2011).
[3] "Nonlinear magnetization dynamics of antiferromagnetic spin resonanceinduced by intense terahertz magnetic field", Y. Mukai, H. Hirori, T. Yamamoto,H. Kageyama and K. Tanaka, New J. Phys. 18 013045 (2016).
[4] "Ultrafast Carrier Dynamics Under High Electric Field in Graphene", S. Tani, F.
Blanchard, and K. Tanaka Phys. Rev. Lett. 109, 166603 (2012).
[5] "High-harmonic generation in graphene enhanced by elliptically polarized light
excitation", N. Yoshikawa, T. Tamaya, and K. Tanaka, Science 356, 736-738(2017).