A research paper by Associate Professor Masaomi Tanaka of Tohoku University (at the time of publication, of the Division of Theoretical Astronomy/Center for Computational Astrophysics of NAOJ) et al. “Kilonova from post-merger ejecta as an optical and near-Infrared counterpart of GW170817” was awarded the 2019 PASJ Excellent Paper Award. This award is given to the author(s) of the most outstanding and inventive paper(s) published by the Publications of the Astronomical Society of Japan (PASJ) during the past 5 years, commending their contributions to the advancement of astronomy. In this paper, it is shown for the first time by observation and simulation that heavy elements are produced in large quantities by a neutron star merger.

Figure 1: Associate Professor Masaomi Tanaka, Tohoku University, gives a lecture to commemorate receiving the PASJ Excellent Paper Award. (Credit: The Astronomical Society of Japan)

On August 17, 2017, the first ever gravitational wave originating from a neutron star merger was observed, and electromagnetic radiation called a "kilonova" was detected from its source, GW170817. A kilonova is a phenomenon in which elements heavier than iron are formed in the material ejected from a neutron star merger and emit electromagnetic waves due to their radioactive decay. Tanaka, the lead author of this paper, used NAOJ's supercomputer ATERUI to simulate the evolution of the luminosity of a kilonova, and compared it with the luminosity data of GW170817 observed by the Subaru Telescope and IRSF. As a result, it was found that more than 10,000 times the mass of the Earth in heavy elements was produced by this neutron star merger.

The Tanaka et al. paper shows that neutron star mergers could be the origin of the heavy elements in the Universe and has greatly advanced our understanding of the origin of the elements. In addition, this research was made possible by multi-messenger astronomy using gravitational waves and electromagnetic waves, and by computational astrophysics using a supercomputer. As of September 1, 2020, the number of citations for the award-winning paper reached 141 (according to NASA ADS), which shows it has had a significant impact on research in this field. The high academic significance of this paper earned it the 2019 PASJ Excellent Paper Award.

The lead author Tanaka says, “On behalf of all the co-authors of the paper, I would like to thank you for this prestigious award. This paper is the result of the work of many people. After the publication of this paper, gravitational waves from neutron star coalescence were detected, and there are more and more opportunities to find out what is happening in the extreme conditions of neutron star mergers and where the heavy elements in the Universe came from. Please look forward to the future developments.”

Tanaka gave a lecture in acceptance of the award on September 9 at the annual autumn meeting of the Astronomical Society of Japan (held online).

Figure 2: Three-color false-color composite images showing the time evolution of the optical and near-infrared counterpart of GW170817 made using data from the Subaru Telescope (z-band, blue) and IRSF (H-band, green; Ks-band, red). (Credit: NAOJ/Nagoya University)

Figure 3: Observations of GW170817 (filled circles) and light curves simulated by ATERUI (solid and dashed lines). The observational data fall close to the solid lines showing simulation results including heavy element formation. The dashed line shows a simulation without heavy element formation. (Credit: NAOJ)

(News release on September 11, 2020)

[Computers Used in This Research]

Tanaka utilized the NAOJ supercomputer ATERUI (Cray XC30) to simulate the evolution of the luminosity of a kilonova. ATERUI had been operated at NAOJ Mizusawa Campus (Oshu, Iwate) until March 2018 with a theoretical peak performance of 1.058 Pflops. (Image Credit: NAOJ)

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