XRISM Spectroscopy of the Fe Kα Emission Line in the Seyfert Active Galactic Nucleus NGC 4151 Reveals the Disk, Broad-line Region, and Torus
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 E. , 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 , Porter Frederick S. , 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 , 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 , Xiang Xin , Minezaki Takeo , Buhariwalla Margaret , Gerolymatou Dimitra , Hagen Scott 
Published: September 2024

Abstract
We present an analysis of the first two XRISM/Resolve spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 4151, obtained in 2023 December. Our work focuses on the nature of the narrow Fe K
α
emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. The total line is found to consist of three components. Even the narrowest component of the line is resolved with evident Fe K
α,1 (6.404 keV) and K
α,2 (6.391 keV) contributions in a 2:1 flux ratio, fully consistent with neutral gas with negligible bulk velocity. Subject to the limitations of our models, the narrowest and intermediate-width components are consistent with emission from optically thin gas, suggesting that they arise in a disk atmosphere and/or wind. Modeling the three line components in terms of Keplerian broadening, they are readily associated with (1) the inner wall of the “torus,” (2) the innermost optical “broad-line region” (or “X-ray BLR”), and (3) a region with a radius of r ≃ 100 GM/c
2 that may signal a warp in the accretion disk. Viable alternative explanations of the broadest component include a fast-wind component and/or scattering; however, we find evidence of variability in the narrow Fe K
α
line complex on timescales consistent with small radii. The best-fit models are statistically superior to simple Voigt functions, but when fit with Voigt profiles the time-averaged lines are consistent with a projected velocity broadening of FWHM

=

1600

−
200

+
400

km

s

−
1

. Overall, the resolution and sensitivity of XRISM show that the narrow Fe K line in AGN is an effective probe of all key parts of the accretion flow, as it is currently understood. We discuss the implications of these findings for our understanding of AGN accretion, future studies with XRISM, and X-ray-based black hole mass measurements.