Gravitational waves radiated from coalescing binary neutron stars was discovered for the first time on August 17, 2017. At the same time, electromagnetic counterparts were detected in a wide range of wavelength from gamma ray to radio. Undoubtedly, the era of multi-messenger gravitational wave astronomy has started. In particular, observations in the visible light and infrared bands provided abundant information about the properties of the released ejecta from the coalescence. The observations were quite consistent with the results based on general relativistic simulations (numerical relativity). Numerical relativity supports the so-called kilonova/macronova hypothesis: a few % solar mass of neutron-rich matter will be released after coalescence and atoms heavier than iron will be rapidly synthesized through the fast neutron capture reaction (r-process), and then the ejecta will shine over several days due to the radioactive decay of the unstable heavy nuclei. In addition to the support of the qualitative picture of kilonova/macronova hypothesis, numerical relativity can actually reproduce how the brightness of electromagnetic wave at each wavelength changes in time. In other words, numerical relativity can provide results quantitatively consistent with observational results.
Sensing this trend of research in advance, from the late 2000s, Dr. Sekiguchi started to construct numerical relativity codes that incorporate the process of neutrino transfer and realistic equation of state of high-density nuclear matter, based on the original development of formulation of the numerical scheme, ahead of the rest of the world (Reference 1). Since the 2010s, he and his collaborators have carried out accurate numerical relativity simulations of neutron star mergers to be quantitatively compared with observations, and gave theoretical predictions, especially about mass ejection (References 2, 3, 4) and heavy element synthesis (Ref. 5). These quantitative predictions gave a theoretical foundation which is crucial for interpreting the electromagnetic wave observations from coalescing binary neutron stars, in particular GW170817.
In order to establish quantitatively reliable predictions, it is essential to incorporate realistic elementary processes into numerical relativity codes. In particular, by the irradiation of high-intensity neutrinos from the massive neutron star formed after the coalescence and by the shock wave heating, the neutron fraction of the ejecta has a wide distribution. In the ejecta with high neutron excess, the r-process proceeds efficiently. As a result, not only elements of mass number 90-190 (e.g., strontium, silver, rare earth etc.) but also even heavier elements with mass number exceeding 190 (e.g., Gold, platinum, uranium) are also synthesized. The resulting abundance ratio becomes close to that of the solar system. Although similar researches on heavy element synthesis at neutron star mergers had been done before, it had been thought difficult to reproduce the solar abundance and explain a large fraction of heavy elements of our universe with binary neutron star mergers because general relativistic effects and neutrino irradiation were not taken into account. It is true that several other collaborators also contributed to the work to obtain the results mentioned above, but Dr. Sekiguchi's contribution is outstanding with regard to the development, execution and interpretation of the numerical relativity calculations with realistic microphysics.
It is almost certain that gravitational wave astronomy and physics will further develop in the future. At that time, the achievement by Dr. Sekiguchi will become an essential theoretical foundation for interpreting observational results, and he himself is also expected to continue to contribute to this research area. Therefore, we conclude it truly appropriate to send Yukawa-Kimura Prize of this year to Dr. Sekiguchi.
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