Diffraction modelling of a 2023 March 5 stellar occultation by subkilometer-sized asteroid (98943) 2001 CC21

Publications of the Astronomical Society of Japan, 2024, 76(5), 940–949 https://doi.org/10.1093/pasj/psae060 Advance access publication date: 2024 July 23 Diffraction modelling of a 2023 March 5 stellar occultation by subkilometer-sized asteroid (98943) 2001 CC21 1 The Hakubi Center/Astronomical Observatory, Graduate School of Science, Kyoto University Kitashirakawa-oiwake-cho, Sakyo-ku, Kyoto, Kyoto 606-8502, Japan 2 University of Occupational and Environmental Health, Japan, 1-1 Iseigaoka, Yahatanishi, Kitakyusyu, Fukuoka 807-8555, Japan 3 Planetary Exploration Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan 4 Saga Hoshizora Astronomy Center, 328 Nishiyoka-cho, Oaza Takataro, Saga, Saga 840-0036, Japan 5 Japan Occultation Information Network (JOIN), Japan 6 Department of Earth Sciences, Okayama University, 3-1-1 Tsushimanaka, Kita, Okayama, Okayama 700-8530, Japan 7 Museum of Astronomical Telescopes, 30-1 Sukemitsu-Higashi, Tawa, Sanuki, Kagawa 769-2306, Japan 8 Astronomy Research Center, Chiba Institute of Technology, 2-17-1 Tsudanuma, Narashino, Chiba 275-0016, Japan 9 Hiroshima Astrophysical Science Center, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan 10 GROUND Geoscience club, Hokkaido University, Kita-10 Nishi-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan 11 Kakogawa Outdoor Education Center, 715-5 Amagahara, Higashikanki-cho, Kakogawa, Hyogo 675-0058, Japan 12 Kuma Kogen Astronomical Observatory, 488 Shimohatanokawa Otsu, Kumakogen, Kamiukena-gun, Ehime, 791-1212, Japan 13 Astronomical Club of Fukuyama, Fukuyama, Hiroshima, Japan 14 Anan Science Center, 8-1 Nagakawa Kamifukui Minami-Kawabuchi, Anan, Tokushima 779-1243, Japan 15 Department of Physics, Graduate School of Advanced Science and Engineering, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan 16 Kougominami, Nishi-ku, Hiroshima, Hiroshima 733-0823, Japan 17 Life Park Kurashiki Science Center, 940 Koshinden, Fukudacho, Kurashiki, Okayama 712-8046, Japan 18 Japanese Association of Accounting and Astronomy, 1-1-1 Nishimachi, Fukuyama, Hiroshima 720-0067, Japan 19 Hattoji Star Watching Group, 1393 Kagami, Yoshinaga-cho, Bizen, Okayama 709-0301, Japan 20 Kaichi, Yao, Osaka 581-0881, Japan 21 Ehime Prefecture Science Museum, 2133-2 Ojoin, Niihama, Ehime 792-0060, Japan 22 Ryuten Astronomical Observatory, 2978-3 Nakaseijitsu, Akaiwa, Okayama 701-2437, Japan 23 NPO KWASAN ASTRO NETWORK, c/o Kwasan Observatory, 17 Kitakazan-ohmine-cho, Yamashina-ku, Kyoto, Kyoto 607-8471 Japan 24 Hokkaido University Astronomy Club, Hokkaido University, Kita-10 Nishi-8, Kita-ku, Sapporo, Hokkaido 060-0810, Japan 25 Daniel Guggenheim School of Aerospace Engineering, Georgia Institute of Technology, North Avenue, Atlanta, GA 30332, USA 26 Institute of Space and Astronautical Science (ISAS), Japan Aerospace Exploration Agency (JAXA), 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa 252-5210, Japan ∗ E-mail: Abstract We present an analysis of a stellar occultation event caused by a near-Earth asteroid (98943) 2001 CC21, an upcoming flyby target in the Hayabusa2 extended mission, on 2023 March 5. To determine the asteroid’s shape from diffraction-affected light curves accurately, we developed a novel data-reduction technique named the Diffracted Occultation’s United Simulator for Highly Informative Transient Explorations (DOUSHITE). Using DOUSHITE-generated synthetic models, we derived constraints on (98943) 2001 CC21’s shadow shape from the single-chord occultation data. Our results suggest a significant elongation of the shadow with an axis ratio of b/a = 0.37 ± 0.09. This shape could be crucial for planning Hayabusa2’s high-speed flyby to optimise the limited imaging opportunities. Keywords: minor planets, asteroids: individual ((98943) 2001 CC21) Received: 2024 April 12; Accepted: 2024 June 16 © The Author(s) 2024. Published by Oxford University Press on behalf of the Astronomical Society of Japan. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. Ko ARIMATSU ,1 ,∗ Fumi YOSHIDA ,2 ,3 Tsutomu HAYAMIZU,4 Miyoshi IDA,5 George L. HASHIMOTO ,6 Takashi ABE,7 Hiroshi AKITAYA ,8 ,3 ,9 Akari ARATANI,10 Hidekazu FUKUDA,11 Yasuhide FUJITA,12 Takao FUJIWARA,6 Toshihiro HORIKAWA,7 Tamio IIHOSHI,13 Kazuyoshi IMAMURA,14 Ryo IMAZAWA ,15 Hisashi KASEBE,5 Ryosuke KAWASAKI,16 Hiroshi KISHIMOTO,5 Kazuhisa MISHIMA,17 Machiko MIYACHI,18 Masanori MIZUTANI,19 Maya NAKAJIMA,6 Hiroyoshi NAKATANI,13 Kazuhiko OKAMURA,7 Misaki OKANOBU,6 Masataka OKUDA,20 Yuji SUZUKI,21 Naoto TATSUMI,22 Masafumi UNO,7 Hidehito YAMAMURA,23 Mikoto YASUE,24 ,10 Hideki YOSHIHARA,5 Masatoshi HIRABAYASHI ,25 and Makoto YOSHIKAWA 26 Publications of the Astronomical Society of Japan (2024), Vol. 76, No. 5 1 Introduction where λ is the wavelength of the observations and is assumed to be λ = 500 nm. Recent developments in stellar astrometry, such as the Gaia catalog (e.g., Gaia Collaboration 2021, 2023), have allowed for unprecedented accuracy in predicting occultation events by kilometer- to subkilometer-sized bodies (e.g., Yoshida et al. 2023). In addition, several observation programmes for small, very distant, and unknown objects will be performed in the near future (Arimatsu et al. 2017, 2019b). However, the sizes of these occulting bodies are close to the Fresnel scale, and the light curves suffer severe diffraction effects. To constrain the shapes of the occultation shadows with limited observational resources, an alternative data reduction technique is necessary. On 2023 March 5, a stellar occultation event by the asteroid (98943) 2001 CC21 occurred in western Japan. (98943) 2001 CC21 is a near-Earth asteroid (NEA) belonging to the Apollo group and has been selected as a flyby target of JAXA’s Hayabusa2 extended mission (Hayabusa2#, Hirabayashi et al. 2021). In 2026 July, the spacecraft will approach the asteroid at a distance of less than 100 km, with a relative velocity of ∼ 5 km s−1 . Due to the limited time available during the flyby, it will be challenging to make a detailed observation plan of the asteroid based on in situ measurements. Accurate shape estimates before the flyby help to plan the proximity observations on the Hayabusa2# extended mission.1 This paper proposes a new data-reduction technique for stellar occultations by small or distant solar system objects. The technique considers the diffraction effect produced by occulting shadows of arbitrary shape in the observed light curves. Additionally, a Bayesian approach is adopted to derive posterior probability distributions for the size, shape, and orientation parameters from the limited datasets. The methodology for modeling diffraction effects is prese (...truncated)