Overionized plasma in the supernova remnant Sagittarius A East anchored by XRISM observations
Authors:   XRISM Collaboration, Audard Marc , Awaki Hisamitsu , Ballhausen Ralf , Bamba Aya , Behar Ehud , Boissay-Malaquin Rozenn , Brenneman Laura , Brown Gregory V , Corrales Lia , Costantini Elisa , Cumbee Renata , Diaz-Trigo Maria , Done Chris , Dotani Tadayasu , Ebisawa Ken , Eckart Megan , Eckert Dominique , Enoto Teruaki , Eguchi Satoshi , Ezoe Yuichiro , Foster Adam , Fujimoto Ryuichi , Fujita Yutaka , Fukazawa Yasushi , Fukushima Kotaro , Furuzawa Akihiro , Gallo Luigi , García Javier A , Gu Liyi , Guainazzi Matteo , Hagino Kouichi , Hamaguchi Kenji , Hatsukade Isamu , Hayashi Katsuhiro , Hayashi Takayuki , Hell Natalie , Hodges-Kluck Edmund , Hornschemeier Ann , Ichinohe Yuto , Ishida Manabu , Ishikawa Kumi , Ishisaki Yoshitaka , Kaastra Jelle , Kallman Timothy , Kara Erin , Katsuda Satoru , Kanemaru Yoshiaki , Kelley Richard , Kilbourne Caroline , Kitamoto Shunji , Kobayashi Shogo , Kohmura Takayoshi , Kubota Aya , Leutenegger Maurice , Loewenstein Michael , Maeda Yoshitomo , Markevitch Maxim , Matsumoto Hironori , Matsushita Kyoko , McCammon Dan , McNamara Brian , Mernier François , Miller Eric D , Miller Jon M , Mitsuishi Ikuyuki , Mizumoto Misaki , Mizuno Tsunefumi , Mori Koji , Mukai Koji , Murakami Hiroshi , Mushotzky Richard , Nakajima Hiroshi , Nakazawa Kazuhiro , Ness Jan-Uwe , Nobukawa Kumiko , Nobukawa Masayoshi , Noda Hirofumi , Odaka Hirokazu , Ogawa Shoji , Ogorzalek Anna , Okajima Takashi , Ota Naomi , Paltani Stephane , Petre Robert , Plucinsky Paul , Scott Porter Frederick , Pottschmidt Katja , Sato Kosuke , Sato Toshiki , Sawada Makoto , Seta Hiromi , Shidatsu Megumi , Simionescu Aurora , Smith Randall , Suzuki Hiromasa , Szymkowiak Andrew , Takahashi Hiromitsu , Takeo Mai , Tamagawa Toru , Tamura Keisuke , Tanaka Takaaki , Tanimoto Atsushi , Tashiro Makoto , Terada Yukikatsu , Terashima Yuichi , Tsuboi Yohko , Tsujimoto Masahiro , Tsunemi Hiroshi , Tsuru Takeshi G , Uchida Hiroyuki , Uchida Nagomi , Uchida Yuusuke , Uchiyama Hideki , Ueda Yoshihiro , Uno Shinichiro , Vink Jacco , Watanabe Shin , Williams Brian J , Yamada Satoshi , Yamada Shinya , Yamaguchi Hiroya , Yamaoka Kazutaka , Yamasaki Noriko , Yamauchi Makoto , Yamauchi Shigeo , Yaqoob Tahir , Yoneyama Tomokage , Yoshida Tessei , Yukita Mihoko , Zhuravleva Irina , Wang Q Daniel , Amano Yuki , Tanaka Kojiro , Narita Takuto , Ohshiro Yuken , Yoshimoto Anje , Aoki Yuma , Balakrishnan Mayura 
Published: December 2024

Sagittarius A East is a supernova remnant with a unique surrounding environment, as it is located in the immediate vicinity of the supermassive black hole at the Galactic center, Sagittarius A$^{*}$. The X-ray emission of the remnant is suspected to show features of overionized plasma, which would require peculiar evolutionary paths. We report on the first observation of Sagittarius A East with the X-Ray Imaging and Spectroscopy Mission (XRISM). Equipped with a combination of a high-resolution microcalorimeter spectrometer and a large field-of-view CCD imager, we for the first time resolved the Fe xxv K-shell lines into fine structure lines and measured the forbidden-to-resonance intensity ratio to be $1.39 \pm 0.12$, which strongly suggests the presence of overionized plasma. We obtained a reliable constraint on the ionization temperature just before the transition into the overionization state, of $\gt\! 4\:$keV. The recombination timescale was constrained to be $\lt\! 8 \times 10^{11} \:$cm$^{-3}\:$s. The small velocity dispersion of $109 \pm 6\:$km$\:$s$^{-1}$ indicates a low Fe ion temperature $\lt\! 8\:$keV and a small expansion velocity $\lt\! 200\:$km$\:$s$^{-1}$. The high initial ionization temperature and small recombination timescale suggest that either rapid cooling of the plasma via adiabatic expansion from dense circumstellar material or intense photoionization by Sagittarius A$^{*}$ in the past may have triggered the overionization.