title: Overionized plasma in the supernova remnant Sagittarius A East anchored by XRISM observations
Creators: name: Boissay-Malaquin, Rozenn
name: Hamaguchi, Kenji
name: Hayashi, Takayuki
name: Mukai, Koji
name: Pottschmidt, Katja
name: Tamura, Keisuke
name: Yaqoob, Tahir
name: XRISM Collaboration
Publisher: name: Oxford University Press
Year: 2024

description: XRISM Collaboration, Marc AUDARD, Hisamitsu AWAKI, Ralf BALLHAUSEN, Aya BAMBA, Ehud BEHAR, Rozenn BOISSAY-MALAQUIN, Laura BRENNEMAN, Gregory V. BROWN, Lia CORRALES, Elisa COSTANTINI, Renata CUMBEE, Maria DIAZ-TRIGO, Chris DONE, Tadayasu DOTANI, Ken EBISAWA, Megan ECKART, Dominique ECKERT, Teruaki ENOTO, Satoshi EGUCHI, Yuichiro EZOE, Adam FOSTER, Ryuichi FUJIMOTO, Yutaka FUJITA, Yasushi FUKAZAWA, Kotaro FUKUSHIMA, Akihiro FURUZAWA, Luigi GALLO, Javier A. GARC虯, Liyi GU, Matteo GUAINAZZI, Kouichi HAGINO, Kenji HAMAGUCHI, Isamu HATSUKADE, Katsuhiro HAYASHI, Takayuki HAYASHI, Natalie HELL, Edmund HODGES-KLUCK, Ann HORNSCHEMEIER, Yuto ICHINOHE, Manabu ISHIDA, Kumi ISHIKAWA, Yoshitaka ISHISAKI, Jelle KAASTRA, Timothy KALLMAN, Erin KARA, Satoru KATSUDA, Yoshiaki KANEMARU, Richard KELLEY, Caroline KILBOURNE, Shunji KITAMOTO, Shogo KOBAYASHI, Takayoshi KOHMURA, Aya KUBOTA, Maurice LEUTENEGGER, Michael LOEWENSTEIN, Yoshitomo MAEDA, Maxim MARKEVITCH, Hironori MATSUMOTO, Kyoko MATSUSHITA, Dan MCCAMMON, Brian MCNAMARA, Fran鏾is MERNIER, Eric D. MILLER, Jon M. MILLER, Ikuyuki MITSUISHI, Misaki MIZUMOTO, Tsunefumi MIZUNO, Koji MORI, Koji MUKAI, Hiroshi MURAKAMI, Richard MUSHOTZKY, Hiroshi NAKAJIMA, Kazuhiro NAKAZAWA, Jan-Uwe NESS, Kumiko NOBUKAWA, Masayoshi NOBUKAWA, Hirofumi NODA, Hirokazu ODAKA, Shoji OGAWA, Anna OGORZALEK, Takashi OKAJIMA, Naomi OTA, Stephane PALTANI, Robert PETRE, Paul PLUCINSKY, Frederick SCOTT PORTER, Katja POTTSCHMIDT, Kosuke SATO, Toshiki SATO, Makoto SAWADA, Hiromi SETA, Megumi SHIDATSU, Aurora SIMIONESCU, Randall SMITH, Hiromasa SUZUKI, Andrew SZYMKOWIAK, Hiromitsu TAKAHASHI, Mai TAKEO, Toru TAMAGAWA, Keisuke TAMURA, Takaaki TANAKA, Atsushi TANIMOTO, Makoto TASHIRO, Yukikatsu TERADA, Yuichi TERASHIMA, Yohko TSUBOI, Masahiro TSUJIMOTO, Hiroshi TSUNEMI, Takeshi G. TSURU, Hiroyuki UCHIDA, Nagomi UCHIDA, Yuusuke UCHIDA, Hideki UCHIYAMA, Yoshihiro UEDA, Shinichiro UNO, Jacco VINK, Shin WATANABE, Brian J. WILLIAMS, Satoshi YAMADA, Shinya YAMADA, Hiroya YAMAGUCHI, Kazutaka YAMAOKA, Noriko YAMASAKI, Makoto YAMAUCHI, Shigeo YAMAUCHI, Tahir YAQOOB, Tomokage YONEYAMA, Tessei YOSHIDA, Mihoko YUKITA, Irina ZHURAVLEVA, Q. Daniel WANG, Yuki AMANO, Kojiro TANAKA, Takuto NARITA, Yuken OHSHIRO, Anje YOSHIMOTO, Yuma AOKI, and Mayura BALAKRISHNAN.
description: Sagittarius燗燛ast 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燗$^{*}$. 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燗燛ast 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燗$^{*}$ in the past may have triggered the overionization.