Hitomi observation of radio galaxy NGC 1275: The first X-ray microcalorimeter spectroscopy of Fe-Kα line emission from an active galactic nucleus
Authors: Aharonian Hitomi Collaboration, Felix , Akamatsu Hiroki , Akimoto Fumie , Allen Steven W , Angelini Lorella , Audard Marc , Awaki Hisamitsu , Axelsson Magnus , Bamba Aya , Bautz Marshall W , Blandford Roger , Brenneman Laura W , Brown Gregory V , Bulbul Esra , Cackett Edward M , Chernyakova Maria , Chiao Meng P , Coppi Paolo S , Costantini Elisa , de Plaa Jelle , de Vries Cor P , den Herder Jan-Willem , Done Chris , Dotani Tadayasu , Ebisawa Ken , Eckart Megan E , Enoto Teruaki , Ezoe Yuichiro , Fabian Andrew C , Ferrigno Carlo , Foster Adam R , Fujimoto Ryuichi , Fukazawa Yasushi , Furuzawa Akihiro , Galeazzi Massimiliano , Gallo Luigi C , Gandhi Poshak , Giustini Margherita , Goldwurm Andrea , Gu Liyi , Guainazzi Matteo , Haba Yoshito , Hagino Kouichi , Hamaguchi Kenji , Harrus Ilana M , Hatsukade Isamu , Hayashi Katsuhiro , Hayashi Takayuki , Hayashida Kiyoshi , Hiraga Junko S , Hornschemeier Ann , Hoshino Akio , Hughes John P , Ichinohe Yuto , Iizuka Ryo , Inoue Hajime , Inoue Yoshiyuki , Ishida Manabu , Ishikawa Kumi , Ishisaki Yoshitaka , Iwai Masachika , Kaastra Jelle , Kallman Tim , Kamae Tsuneyoshi , Kataoka Jun , Katsuda Satoru , Kawai Nobuyuki , Kelley Richard L , Kilbourne Caroline A , Kitaguchi Takao , Kitamoto Shunji , Kitayama Tetsu , Kohmura Takayoshi , Kokubun Motohide , Koyama Katsuji , Koyama Shu , Kretschmar Peter , Krimm Hans A , Kubota Aya , Kunieda Hideyo , Laurent Philippe , Lee Shiu-Hang , Leutenegger Maurice A , Limousin Olivier O , Loewenstein Michael , Long Knox S , Lumb David , Madejski Greg , Maeda Yoshitomo , Maier Daniel , Makishima Kazuo , Markevitch Maxim , Matsumoto Hironori , Matsushita Kyoko , McCammon Dan , McNamara Brian R , Mehdipour Missagh , Miller Eric D , Miller Jon M , Mineshige Shin , Mitsuda Kazuhisa , Mitsuishi Ikuyuki , Miyazawa Takuya , Mizuno Tsunefumi , Mori Hideyuki , Mori Koji , Mukai Koji , Murakami Hiroshi , Mushotzky Richard F , Nakagawa Takao , Nakajima Hiroshi , Nakamori Takeshi , Nakashima Shinya , Nakazawa Kazuhiro , Nobukawa Kumiko K , Nobukawa Masayoshi , Noda Hirofumi , Odaka Hirokazu , Ohashi Takaya , Ohno Masanori , Okajima Takashi , Ota Naomi , Ozaki Masanobu , Paerels Frits , Paltani Stéphane , Petre Robert , Pinto Ciro , Porter Frederick S , Pottschmidt Katja , Reynolds Christopher S , Safi-Harb Samar , Saito Shinya , Sakai Kazuhiro , Sasaki Toru , Sato Goro , Sato Kosuke , Sato Rie , Sawada Makoto , Schartel Norbert , Serlemitsos Peter J , Seta Hiromi , Shidatsu Megumi , Simionescu Aurora , Smith Randall K , Soong Yang , Stawarz Łukasz , Sugawara Yasuharu , Sugita Satoshi , Szymkowiak Andrew , Tajima Hiroyasu , Takahashi Hiromitsu , Takahashi Tadayuki , Takeda Shin’ichiro , Takei Yoh , Tamagawa Toru , Tamura Takayuki , Tanaka Takaaki , Tanaka Yasuo , Tanaka Yasuyuki T , Tashiro Makoto S , Tawara Yuzuru , Terada Yukikatsu , Terashima Yuichi , Tombesi Francesco , Tomida Hiroshi , Tsuboi Yohko , Tsujimoto Masahiro , Tsunemi Hiroshi , Tsuru Takeshi Go , Uchida Hiroyuki , Uchiyama Hideki , Uchiyama Yasunobu , Ueda Shutaro , Ueda Yoshihiro , Uno Shin’ichiro , Urry C Megan , Ursino Eugenio , Watanabe Shin , Werner Norbert , Wilkins Dan R , Williams Brian J , Yamada Shinya , Yamaguchi Hiroya , Yamaoka Kazutaka , Yamasaki Noriko Y , Yamauchi Makoto , Yamauchi Shigeo , Yaqoob Tahir , Yatsu Yoichi , Yonetoku Daisuke , Zhuravleva Irina , Zoghbi Abderahmen , Kawamuro Taiki 
Published: April 2018

The origin of the narrow Fe-Kα fluorescence line at 6.4 keV from active galactic nuclei has long been under debate; some of the possible sites are the outer accretion disk, the broad line region, a molecular torus, or interstellar/intracluster media. In 2016 February–March, we performed the first X-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS) on board the Hitomi satellite of the Fanaroff–Riley type I radio galaxy NGC 1275 at the center of the Perseus cluster of galaxies. With the high-energy resolution of ∼5 eV at 6 keV achieved by Hitomi/SXS, we detected the Fe-Kα line with ∼5.4 σ significance. The velocity width is constrained to be 500–1600 km s−1 (FWHM for Gaussian models) at 90% confidence. The SXS also constrains the continuum level from the NGC 1275 nucleus up to ∼20 keV, giving an equivalent width of ∼20 eV for the 6.4 keV line. Because the velocity width is narrower than that of the broad Hα line of ∼2750 km s−1, we can exclude a large contribution to the line flux from the accretion disk and the broad line region. Furthermore, we performed pixel map analyses on the Hitomi/SXS data and image analyses on the Chandra archival data, and revealed that the Fe-Kα line comes from a region within ∼1.6 kpc of the NGC 1275 core, where an active galactic nucleus emission dominates, rather than that from intracluster media. Therefore, we suggest that the source of the Fe-Kα line from NGC 1275 is likely a low-covering-fraction molecular torus or a rotating molecular disk which probably extends from a parsec to hundreds of parsecs scale in the active galactic nucleus system.