The gravitational wave event GW170817 from a merger of two neutron stars discovered on August 17, 2017 was a historic milestone that marked the beginning of the new era of multi-messenger astrophysics. Not only was this the first discovery of a binary neutron star merger, but also it involved many physicists and astronomers around the world, as the electromagnetic counterparts were observed across the entire wavelength range from radio to gamma-rays. The observation of various electromagnetic counterparts has brought a wealth of unprecedented knowledge and significant progress. In particular, radiation known as a kilonova/macronova has been observed in the ultraviolet, visible, and infrared bands, and it is now thought to shine due to the radioactive decay of heavy elements synthesized in the neutron-rich ejecta from the neutron star merger. This was an important advance in our understanding of the origin of heavy elements. In addition, the observations of X-rays, gamma-rays, and radio waves have conclusively shown that gamma-ray bursts (of short duration) occurred during the coalescence, proving one of the origins of short gamma-ray bursts, which had been only a hypothesis for many years. It should also be noted that the Hubble constant was measured in a completely independent manner by combining gravitational wave observations with high-resolution radio observations of the afterglow of the gamma-ray burst jet.
Dr. Hotokezaka is one of the world leaders in pioneering theoretical research on electromagnetic counterparts and achieving important results ahead of the dawn of this era of multi-messenger astrophysics. He has written a number of important papers, three themes of which are listed below:
1. Mass ejection due to binary neutron star mergers (Ref.[1])Dr. Hotokezaka pioneered the use of numerical relativity to study mass ejection phenomena associated with binary neutron star mergers, providing a foundational prediction for interpreting the observations of the electromagnetic counterpart of GW170817. The mass ejection not only leads to the origin of heavy elements per se, but also to kilonova radiation through radioactive decay. In addition, the gamma-ray burst jet must penetrate this ejecta before it emits gamma-rays. Therefore, the mass ejection phenomena are the starting point for understanding electromagnetic counterparts, and this study, the first to be calculated using numerical relativity, is extremely important.
The optical and radioactive decay properties of kilonovae that glow in the decay of r-process elements were investigated. In GRB 130603B, it was showed that the infrared counterpart can be interpreted naturally by the kilonova model, providing a theoretical basis for interpreting GW170817. The nature of the radioactive decay of kilonova and the thermalization process of the radiation were also investigated. Using these results and atomic structure calculations, he is also the first in the world to calculate the emission line spectra and their time evolution due to heavy elements in the late kilonova period. In addition, his work on estimating the masses ejected from neutron star mergers using astronomical and geological data is highly original.
Dr. Hotokezaka systematically examined possible radio counterparts prior to the discovery of GW170817 and proposed a strategy to detect them. At the time of the discovery of GW170817, he was involved in the discovery of the radio counterpart by the Very Large Array (VLA) and led the theoretical interpretation of the observations. In particular, since the VLBI observed superluminal motion of the afterglow of the jet, it was shown that the accuracy of the measurement of the Hubble constant can be greatly improved by measuring the viewing angle to the jet with high precision. This research has had a significant impact not only on astrophysics but also on cosmology.
These series of studies are fundamental to history. His contributions to the study of binary neutron star mergers and multi-messenger astrophysics are very wide-ranging and original. It is also noteworthy that he not only conducts theoretical research, but also works closely with first-class experimentalists to conduct research to verify the theories he has developed. As the sensitivity of the gravitational wave telescope improves in the future, observations of multi-messenger astronomical phenomena will undoubtedly grow, and his future activities are highly promising. For these reasons, we judged Dr. Hotokezaka to be a worthy recipient of the Yukawa-Kimura Prize.