Division of Theoretical Astronomy, National Astronomical Observatory of Japan

Two Researchers in the Division of Theoretical Astronomy won the MEXT Young Scientists’ Prize

Two researchers in the Division of Theoretical Astronomy, Dr. Masaomi Tanaka Assistant Professor (*1) and Dr. Seiji Zenitani NAOJ Fellow, received the 2016 Young Scientists’ Prize from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan. The commendation ceremony was held on April 20, 2016 in Tokyo. Dr. Tanaka was honored for “Theoretical and Observational Research on cosmic Explosions”, and Dr. Zenitani was honored for “Numerical Modeling of Relativistic Magnetic Reconnection.”

*1: Assistant Professor at Graduate University for Advanced Studies (SOKENDAI), Department of Astronomical Science

Photo: Dr. Masaomi Tanaka (left) and Dr. Seiji Zenitani (right) at the awards ceremony (April 20, 2016). The center is Dr. Masao Saito (Associate professor, Director of the Nobeyama Radio Observatory) who recieved the Prizes for Science and Technology, Development Category. (Credit: NAOJ)

Dr. Masaomi Tanaka (Assistant Professor): “Theoretical and Observational Research on Cosmic Explosions”

Various kinds of explosive phenomena exist in our Universe. One of the most exciting is the merger of two neutron stars. Neutron star mergers are known to be strong gravitational waves sources; they are among the most important targets for gravitational-wave telescopes. Since the exact position of a gravitational-wave source cannot be accurately determined from the detection of gravitational waves alone, it is important to be able to identify their electromagnetic counterparts.
Dr. Masaomi Tanaka, together with his colleagues, performed detailed radiative transfer simulations with realistic element abundances to determine the electromagnetic signals we should expect from neutron star mergers. A gamma-ray burst observed in 2013 contained observational signatures which had been predicted by Dr. Tanaka’s simulations. This gamma-ray burst is thought to have been generated by a neutron star merger, so this observation proved the validity of these theoretical models.
This research will provide a guide for observations searching for the electromagnetic counterparts of gravitational wave sources, and thus make large contributions to “multi-messenger astronomy.” Tanaka also participates in observational campaigns searching for gravitational wave sources.

Movie 1: Numerical simulation of electromagnetic emissions from neutron star mergers. This simulation was performed using CfCA/NAOJ supercomputer “ATERUI.”(Copyright© Masaomi Tanaka)

Dr. Tanaka has also been researching supernova explosions through theoretical simulations and observations. In 2012, he showed that supernova explosions have three-dimensional, clumpy structures (Subaru Telescope press release). He also proposed methods to discover supernovae in the high-redshift Universe, or even the “first supernovae” in the Universe. Based on these ideas, he is now working on observational projects using the Subaru Telescope.
By combining theoretical and observational approaches, Dr. Tanaka has contributed to our understanding of explosive phenomena in the Universe. He was awarded the Young Scientists’ Prize for these achievements. “It is a great honor for me to receive this prize,” said Dr. Tanaka. “I am grateful to all of my collaborators for working together with me on theoretical and observational research of various explosive phenomena. I believe that gravitational wave astronomy will dramatically accelerate the progress of neutron star merger research, and that powerful new surveys will advance supernova research to the next stage. Encouraged by this prize, I will keep tackling the mysteries of these explosive phenomena in our Universe.”

Dr. Seiji Zenitani (NAOJ fellow): “Numerical Modeling of Relativistic Magnetic Reconnection”

Magnetic reconnection, the topological re-arrangement of magnetic field lines, is an important phenomenon in space plasmas. Traditionally, the reconnection process has been studied in solar and magnetospheric physics, but it has also attracted growing attention in the plasma environments surrounding high-energy astrophysical objects, such as pulsars and gamma-ray-burst sources. These plasmas are so exotic that relativity needs to be incorporated.
Recognizing the potential role of magnetic reconnection at these sites, Dr. Seiji Zenitani has investigated the basic properties of relativistic magnetic reconnection by using various simulation methods, many of which were highly developed in solar and magnetospheric physics. He carried out the first particle-in-cell (PIC) simulations of relativistic magnetic reconnection in an electron-positron pair plasma. This simulation revealed that magnetic reconnection is a powerful particle accelerator. In addition, he developed a numerical code to simulate fast plasma flowing at speeds near the speed of light. Using this code, he clarified the large-scale properties of relativistic magnetic reconnection. Based on these results, relativistic magnetic reconnection has been discussed as a likely candidate for the mechanism responsible for the gamma-ray flares that were discovered in the Crab Nebula in 2011.

Movie 2: Simulation results for the jet-front structure in relativistic magnetic reconnection. The origin (0, 0) is taken to be at the reconnection point located far outside of the figure. A violent reconnection jet from the left hits the ambient medium, creating beautiful diamond-shaped structures.(Copyright© Seiji Zenitani)

In addition, inspired by relativistic theories, he proposed a new theoretical method to diagonize magnetic reconnection data. In 2015, NASA launched the Magnetospheric MultiScale (MMS) spacecraft to observe near-Earth reconnection sites. It is expected that the new diagnosis will help scientists analyze MMS data.
Dr. Zenitani’s work provides a basis to discuss relativistic magnetic reconnection in high-energy astrophysics. His work also contributes to our understanding of magnetic reconnection in a wide range of plasma environments. Dr. Zenitani says, “Receiving the Young Scientists’ Prize is a great honor for me. I really appreciate my collaborators, colleagues, and NAOJ staff for their continuous support and encouragement. Recently, there is strong interest in magnetic reconnection in high-energy astrophysics. In the Earth’s magnetosphere, the new spacecraft has started to deliver amazing data related to magnetic reconnection. I’ll continue my work on magnetic reconnection, connecting the two exciting fields of high-energy astrophysics and magnetospheric physics.”

This award, the Young Scientists’ Prize, is given to researchers younger than 40 years old who have accomplished remarkable achievements showing great aptitude for exploratory or creative research with unique ideas in science and technology.

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