Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer The corresponding authors are Masahiro Tsujimoto, Koji Mori, Shiu-Hang Lee, Hiroya Yamaguchi, Nozomu Tominaga, Takashi J. Moriya, Toshiki Sato, Cor de Vries, and Ryo Iizuka

Search for Thermal X-ray Features from the Crab nebula with Hitomi Soft X-ray Spectrometer thanks: The corresponding authors are Masahiro Tsujimoto, Koji Mori, Shiu-Hang Lee, Hiroya Yamaguchi, Nozomu Tominaga, Takashi J. Moriya, Toshiki Sato, Cor de Vries, and Ryo Iizuka

Hitomi Collaboration    Felix Aharonian11affiliation: Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland    Hiroki Akamatsu22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Fumie Akimoto33affiliation: Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8601    Steven W. Allen44affiliation: Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA 55affiliation: Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 66affiliation: SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA    Lorella Angelini77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Marc Audard88affiliation: Department of Astronomy, University of Geneva, ch. d’Écogia 16, CH-1290 Versoix, Switzerland    Hisamitsu Awaki99affiliation: Department of Physics, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577    Magnus Axelsson1010affiliation: Department of Physics and Oskar Klein Center, Stockholm University, 106 91 Stockholm, Sweden    Aya Bamba1111affiliation: Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 1212affiliation: Research Center for the Early Universe, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033    Marshall W. Bautz1313affiliation: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA    Roger Blandford44affiliation: Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA 55affiliation: Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA 66affiliation: SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA    Laura W. Brenneman1414affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA    Greg V. Brown1515affiliation: Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA 94550, USA    Esra Bulbul1313affiliation: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA    Edward M. Cackett1616affiliation: Department of Physics and Astronomy, Wayne State University, 666 W. Hancock St, Detroit, MI 48201, USA    Maria Chernyakova11affiliation: Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland    Meng P. Chiao77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Paolo S. Coppi1717affiliation: Department of Physics, Yale University, New Haven, CT 06520-8120, USA 1818affiliation: Department of Astronomy, Yale University, New Haven, CT 06520-8101, USA    Elisa Costantini22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Jelle de Plaa22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Cor P. de Vries22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Jan-Willem den Herder22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Chris Done1919affiliation: Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK    Tadayasu Dotani2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Ken Ebisawa2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Megan E. Eckart77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Teruaki Enoto2121affiliation: Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502 2222affiliation: The Hakubi Center for Advanced Research, Kyoto University, Kyoto 606-8302    Yuichiro Ezoe2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397    Andrew C. Fabian2424affiliation: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK    Carlo Ferrigno88affiliation: Department of Astronomy, University of Geneva, ch. d’Écogia 16, CH-1290 Versoix, Switzerland    Adam R. Foster1414affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA    Ryuichi Fujimoto2525affiliation: Faculty of Mathematics and Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192    Yasushi Fukazawa2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Akihiro Furuzawa2727affiliation: Fujita Health University, Toyoake, Aichi 470-1192    Massimiliano Galeazzi2828affiliation: Physics Department, University of Miami, 1320 Campo Sano Dr., Coral Gables, FL 33146, USA    Luigi C. Gallo2929affiliation: Department of Astronomy and Physics, Saint Mary’s University, 923 Robie Street, Halifax, NS, B3H 3C3, Canada    Poshak Gandhi3030affiliation: Department of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, UK    Margherita Giustini22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Andrea Goldwurm3131affiliation: Laboratoire APC, 10 rue Alice Domon et Léonie Duquet, 75013 Paris, France 3232affiliation: CEA Saclay, 91191 Gif sur Yvette, France    Liyi Gu22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Matteo Guainazzi3333affiliation: European Space Research and Technology Center, Keplerlaan 1 2201 AZ Noordwijk, The Netherlands    Yoshito Haba3434affiliation: Department of Physics and Astronomy, Aichi University of Education, 1 Hirosawa, Igaya-cho, Kariya, Aichi 448-8543    Kouichi Hagino2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Kenji Hamaguchi77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 3535affiliation: Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA    Ilana M. Harrus77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 3535affiliation: Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA    Isamu Hatsukade3636affiliation: Department of Applied Physics and Electronic Engineering, University of Miyazaki, 1-1 Gakuen Kibanadai-Nishi, Miyazaki, 889-2192    Katsuhiro Hayashi2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Takayuki Hayashi3737affiliation: Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602    Kiyoshi Hayashida3838affiliation: Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043    Junko S. Hiraga3939affiliation: Department of Physics, Kwansei Gakuin University, 2-1 Gakuen, Sanda, Hyogo 669-1337    Ann Hornschemeier77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Akio Hoshino4040affiliation: Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501    John P. Hughes4141affiliation: Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Road, Piscataway, NJ 08854, USA    Yuto Ichinohe2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397    Ryo Iizuka2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Hajime Inoue4242affiliation: Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506    Yoshiyuki Inoue2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Manabu Ishida2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Kumi Ishikawa2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Yoshitaka Ishisaki2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397    Jelle Kaastra22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands 4343affiliation: Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands    Tim Kallman77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Tsuneyoshi Kamae1111affiliation: Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033    Jun Kataoka4444affiliation: Research Institute for Science and Engineering, Waseda University, 3-4-1 Ohkubo, Shinjuku, Tokyo 169-8555    Satoru Katsuda4545affiliation: Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551    Nobuyuki Kawai4646affiliation: Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550    Richard L. Kelley77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Caroline A. Kilbourne77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Takao Kitaguchi2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Shunji Kitamoto4040affiliation: Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501    Tetsu Kitayama4747affiliation: Department of Physics, Toho University, 2-2-1 Miyama, Funabashi, Chiba 274-8510    Takayoshi Kohmura4848affiliation: Department of Physics, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba, 278-8510    Motohide Kokubun2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Katsuji Koyama4949affiliation: Department of Physics, Kyoto University, Kitashirakawa-Oiwake-Cho, Sakyo, Kyoto 606-8502    Shu Koyama2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Peter Kretschmar5050affiliation: European Space Astronomy Center, Camino Bajo del Castillo, s/n., 28692 Villanueva de la Cañada, Madrid, Spain    Hans A. Krimm5151affiliation: Universities Space Research Association, 7178 Columbia Gateway Drive, Columbia, MD 21046, USA 5252affiliation: National Science Foundation, 4201 Wilson Blvd, Arlington, VA 22230, USA    Aya Kubota5353affiliation: Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasaku, Minuma-ku, Saitama, Saitama 337-8570    Hideyo Kunieda3737affiliation: Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602    Philippe Laurent3131affiliation: Laboratoire APC, 10 rue Alice Domon et Léonie Duquet, 75013 Paris, France 3232affiliation: CEA Saclay, 91191 Gif sur Yvette, France    Shiu-Hang Lee2121affiliation: Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502    Maurice A. Leutenegger77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Olivier O. Limousin3232affiliation: CEA Saclay, 91191 Gif sur Yvette, France    Michael Loewenstein77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Knox S. Long5454affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA    David Lumb3333affiliation: European Space Research and Technology Center, Keplerlaan 1 2201 AZ Noordwijk, The Netherlands    Greg Madejski44affiliation: Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA    Yoshitomo Maeda2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Daniel Maier3131affiliation: Laboratoire APC, 10 rue Alice Domon et Léonie Duquet, 75013 Paris, France 3232affiliation: CEA Saclay, 91191 Gif sur Yvette, France    Kazuo Makishima5555affiliation: Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198    Maxim Markevitch77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Hironori Matsumoto3838affiliation: Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043    Kyoko Matsushita5656affiliation: Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601    Dan McCammon5757affiliation: Department of Physics, University of Wisconsin, Madison, WI 53706, USA    Brian R. McNamara5858affiliation: Department of Physics and Astronomy, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L 3G1, Canada    Missagh Mehdipour22affiliation: SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands    Eric D. Miller1313affiliation: Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA    Jon M. Miller5959affiliation: Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA    Shin Mineshige2121affiliation: Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502    Kazuhisa Mitsuda2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Ikuyuki Mitsuishi3737affiliation: Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602    Takuya Miyazawa6060affiliation: Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son Okinawa, 904-0495    Tsunefumi Mizuno2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Hideyuki Mori77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Koji Mori3636affiliation: Department of Applied Physics and Electronic Engineering, University of Miyazaki, 1-1 Gakuen Kibanadai-Nishi, Miyazaki, 889-2192    Koji Mukai77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 3535affiliation: Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA    Hiroshi Murakami6161affiliation: Faculty of Liberal Arts, Tohoku Gakuin University, 2-1-1 Tenjinzawa, Izumi-ku, Sendai, Miyagi 981-3193    Richard F. Mushotzky6262affiliation: Department of Astronomy, University of Maryland, College Park, MD 20742, USA    Takao Nakagawa2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Hiroshi Nakajima3838affiliation: Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043    Takeshi Nakamori6363affiliation: Faculty of Science, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata, Yamagata 990-8560    Shinya Nakashima5555affiliation: Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198    Kazuhiro Nakazawa1111affiliation: Department of Physics, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033    Kumiko K. Nobukawa6464affiliation: Department of Physics, Nara Women’s University, Kitauoyanishi-machi, Nara, Nara 630-8506    Masayoshi Nobukawa6565affiliation: Department of Teacher Training and School Education, Nara University of Education, Takabatake-cho, Nara, Nara 630-8528    Hirofumi Noda6666affiliation: Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, 6-3 Aramakiazaaoba, Aoba-ku, Sendai, Miyagi 980-8578 6767affiliation: Astronomical Institute, Tohoku University, 6-3 Aramakiazaaoba, Aoba-ku, Sendai, Miyagi 980-8578    Hirokazu Odaka66affiliation: SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA    Takaya Ohashi2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397    Masanori Ohno2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Takashi Okajima77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Naomi Ota6464affiliation: Department of Physics, Nara Women’s University, Kitauoyanishi-machi, Nara, Nara 630-8506    Masanobu Ozaki2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Frits Paerels6868affiliation: Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA    Stéphane Paltani88affiliation: Department of Astronomy, University of Geneva, ch. d’Écogia 16, CH-1290 Versoix, Switzerland    Robert Petre77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Ciro Pinto2424affiliation: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge, CB3 0HA, UK    Frederick S. Porter77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Katja Pottschmidt77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 3535affiliation: Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA    Christopher S. Reynolds6262affiliation: Department of Astronomy, University of Maryland, College Park, MD 20742, USA    Samar Safi-Harb6969affiliation: Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB R3T 2N2, Canada    Shinya Saito4040affiliation: Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501    Kazuhiro Sakai77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Toru Sasaki5656affiliation: Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601    Goro Sato2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Kosuke Sato5656affiliation: Department of Physics, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601    Rie Sato2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Toshiki Sato2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397 2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Makoto Sawada7070affiliation: Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258    Norbert Schartel5050affiliation: European Space Astronomy Center, Camino Bajo del Castillo, s/n., 28692 Villanueva de la Cañada, Madrid, Spain    Peter J. Serlemtsos77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Hiromi Seta2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397    Megumi Shidatsu5555affiliation: Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198    Aurora Simionescu2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Randall K. Smith1414affiliation: Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA    Yang Soong77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Łukasz Stawarz7171affiliation: Astronomical Observatory of Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland    Yasuharu Sugawara2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Satoshi Sugita4646affiliation: Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550    Andrew Szymkowiak1717affiliation: Department of Physics, Yale University, New Haven, CT 06520-8120, USA    Hiroyasu Tajima7272affiliation: Institute for Space-Earth Environmental Research, Nagoya University, Furo-cho, Chikusa-ku, Aichi 464-8601    Hiromitsu Takahashi2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Tadayuki Takahashi2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Shiníchiro Takeda6060affiliation: Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son Okinawa, 904-0495    Yoh Takei2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Toru Tamagawa5555affiliation: Institute of Physical and Chemical Research, 2-1 Hirosawa, Wako, Saitama 351-0198    Takayuki Tamura2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Takaaki Tanaka4949affiliation: Department of Physics, Kyoto University, Kitashirakawa-Oiwake-Cho, Sakyo, Kyoto 606-8502    Yasuo Tanaka7373affiliation: Max Planck Institute for extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching , Germany    Yasuyuki T. Tanaka2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Makoto S. Tashiro7474affiliation: Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama, 338-8570    Yuzuru Tawara3737affiliation: Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602    Yukikatsu Terada7474affiliation: Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama, 338-8570    Yuichi Terashima99affiliation: Department of Physics, Ehime University, Bunkyo-cho, Matsuyama, Ehime 790-8577    Francesco Tombesi77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA 6262affiliation: Department of Astronomy, University of Maryland, College Park, MD 20742, USA    Hiroshi Tomida2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Yohko Tsuboi4545affiliation: Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551    Masahiro Tsujimoto2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Hiroshi Tsunemi3838affiliation: Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043    Takeshi Go Tsuru4949affiliation: Department of Physics, Kyoto University, Kitashirakawa-Oiwake-Cho, Sakyo, Kyoto 606-8502    Hiroyuki Uchida4949affiliation: Department of Physics, Kyoto University, Kitashirakawa-Oiwake-Cho, Sakyo, Kyoto 606-8502    Hideki Uchiyama7575affiliation: Faculty of Education, Shizuoka University, 836 Ohya, Suruga-ku, Shizuoka 422-8529    Yasunobu Uchiyama4040affiliation: Department of Physics, Rikkyo University, 3-34-1 Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501    Shutaro Ueda2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Yoshihiro Ueda2121affiliation: Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502    Shiníchiro Uno7676affiliation: Faculty of Health Sciences, Nihon Fukushi University , 26-2 Higashi Haemi-cho, Handa, Aichi 475-0012    C. Megan Urry1717affiliation: Department of Physics, Yale University, New Haven, CT 06520-8120, USA    Eugenio Ursino2828affiliation: Physics Department, University of Miami, 1320 Campo Sano Dr., Coral Gables, FL 33146, USA    Shin Watanabe2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Norbert Werner7777affiliation: MTA-Eötvös University Lendület Hot Universe Research Group, Pázmány Péter sétány 1/A, Budapest, 1117, Hungary 7878affiliation: Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2, Brno, 611 37, Czech Republic 2626affiliation: School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima 739-8526    Dan R. Wilkins44affiliation: Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA    Brian J. Williams5454affiliation: Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA    Shinya Yamada2323affiliation: Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397    Hiroya Yamaguchi77affiliation: NASA, Goddard Space Flight Center, 8800 Greenbelt Road, Greenbelt, MD 20771, USA    Kazutaka Yamaoka3737affiliation: Department of Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Aichi 464-8602    Noriko Y. Yamasaki2020affiliation: Japan Aerospace Exploration Agency, Institute of Space and Astronautical Science, 3-1-1 Yoshino-dai, Chuo-ku, Sagamihara, Kanagawa 252-5210    Makoto Yamauchi3636affiliation: Department of Applied Physics and Electronic Engineering, University of Miyazaki, 1-1 Gakuen Kibanadai-Nishi, Miyazaki, 889-2192    Shigeo Yamauchi6464affiliation: Department of Physics, Nara Women’s University, Kitauoyanishi-machi, Nara, Nara 630-8506    Tahir Yaqoob3535affiliation: Department of Physics, University of Maryland Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USA    Yoichi Yatsu4646affiliation: Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550    Daisuke Yonetoku2525affiliation: Faculty of Mathematics and Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192    Irina Zhuravleva44affiliation: Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, 452 Lomita Mall, Stanford, CA 94305, USA 55affiliation: Department of Physics, Stanford University, 382 Via Pueblo Mall, Stanford, CA 94305, USA    Abderahmen Zoghbi5959affiliation: Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA    Nozomu Tominaga8181affiliation: Department of Physics, Faculty of Science and Engineering, Konan University, 8-9-1 Okamoto, Kobe, Hyogo 658-8501 8282affiliation: Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583    Takashi J. Moriya8383affiliation: National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588 tsujimot@astro.isas.jaxa.jp, mori@astro.miyazaki-u.ac.jp
Abstract

The Crab nebula originated from a core-collapse supernova (SN) explosion observed in 1054 A. D. When viewed as a supernova remnant (SNR), it has an anomalously low observed ejecta mass and kinetic energy for an Fe-core collapse SN. Intensive searches were made for a massive shell that solves this discrepancy, but none has been detected. An alternative idea is that the SN 1054 is an electron-capture (EC) explosion with a lower explosion energy by an order of magnitude than Fe-core collapse SNe. In the X-rays, imaging searches were performed for the plasma emission from the shell in the Crab outskirts to set a stringent upper limit to the X-ray emitting mass. However, the extreme brightness of the source hampers access to its vicinity. We thus employed spectroscopic technique using the X-ray micro-calorimeter onboard the Hitomi satellite. By exploiting its superb energy resolution, we set an upper limit for emission or absorption features from yet undetected thermal plasma in the 2–12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data. By assembling these results, a new upper limit was obtained for the X-ray plasma mass of for a wide range of assumed shell radius, size, and plasma temperature both in and out of the collisional equilibrium. To compare with the observation, we further performed hydrodynamic simulations of the Crab SNR for two SN models (Fe-core versus EC) under two SN environments (uniform ISM versus progenitor wind). We found that the observed mass limit can be compatible with both SN models if the SN environment has a low density of 0.03 cm (Fe core) or 0.1 cm (EC) for the uniform density, or a progenitor wind density somewhat less than that provided by a mass loss rate of  yr at 20 km s for the wind environment.

\Received

2017/06/04 \Accepted2017/06/30 \jyear2017

7979affiliationtext: Department of Physics and Astronomy, University of Utah, 115 South 1400 East, Salt Lake City, Utah 84112, USA8080affiliationtext: The Johns Hopkins University, Homewood Campus, Baltimore, MD 21218, USA\KeyWords

ISM: supernova remnants — Instrumentation: spectrographs — ISM individual (Crab nebula) — Methods: observational

1 Introduction

Out of some 400111See http://www.physics.umanitoba.ca/snr/SNRcat/ for the high-energy catalogues of SNRs and the latest statistics. Galactic supernova remnants (SNRs) detected in the X-rays and -rays (Ferrand & Safi-Harb, 2012), about 10% of them lack shells, which is one of the defining characteristics of SNRs. They are often identified instead as pulsar wind nebulae (PWNe), systems that are powered by the rotational energy loss of a rapidly rotating neutron star generated as a consequence of a core-collapse supernova (SN) explosion.

The lack of a shell in these sources deserves wide attention, since it is a key to unveiling the causes behind the variety of observed phenomena in SNRs. In this pursuit, it is especially important to interpret in the context of the evolution from SNe to SNRs, not just a taxonomy of SNRs. Observed results of SNRs do exhibit imprints of their progenitors, explosion mechanisms, and surrounding environment (Hughes et al., 1995; Yamaguchi et al., 2014a). Recent rapid progress in simulation studies of the stellar evolution of progenitors, SN explosions, and hydrodynamic development of SNRs makes it possible to gain insights about SNe from SNR observations.

The Crab nebula is one such source. It is an observational standard for X-ray and -ray flux and time (Kirsch et al., 2005; Kaastra et al., 2009; Weisskopf et al., 2010; Madsen et al., 2015; Jahoda et al., 2006; Terada et al., 2008). As a PWN, the Crab exhibits typical X-ray and -ray luminosities for its spin-down luminosity (Possenti et al., 2002; Kargaltsev et al., 2012; Mattana et al., 2009) and a typical morphology (Ng & Romani, 2008; Bamba et al., 2010). It also played many iconic roles in the history of astronomy, such as giving observational proof (Staelin & Reifenstein, 1968; Lovelace et al., 1968) for the birth of a neutron star in SN explosions (Baade & Zwicky, 1934) and linking modern and ancient astronomy by its association with a historical SN in 1054 documented primarily in Oriental records (Stephenson & Green, 2002; Lundmark, 1921; Rudie et al., 2008).

This astronomical icon, however, is known to be anomalous when viewed as an SNR. Besides having no detected shell, it has an uncomfortably small observed ejecta mass of 4.6 1.8 (Fesen et al., 1997), kinetic energy of erg (Davidson & Fesen, 1985), and maximum velocity of only 2,500 km s (Sollerman et al., 2000), all of which are far below the values expected for a typical core-collapse SN.

One idea to reconcile this discrepancy is that there is a fast and thick shell yet to be detected, which carries a significant fraction of the mass and kinetic energy (Chevalier, 1977). If the free expansion velocity is 10 km s, the shell radius has grown to 10 pc over 10 yr. Intensive attempts were made to detect such a shell in the radio (Frail et al., 1995), H (Tziamtzis et al., 2009), and X-rays (Mauche & Gorenstein, 1985; Predehl & Schmitt, 1995; Seward et al., 2006), but without success.

Another idea is that the SN explosion was indeed anomalous to begin with. Nomoto et al. (1982) proposed that SN 1054 was an electron-capture (EC) SN, which is caused by the endothermic reaction of electrons captured in an O-Ne-Mg core, in contrast to the photo-dissociation in an Fe core for the normal core-collapse SN. EC SNe are considered to be caused by an intermediate (8–10 ) mass progenitor in the asymptotic giant branch (AGB) phase. Simulations based on the first principle calculation (Kitaura et al., 2006; Janka et al., 2008) show that an explosion takes place with a small energy of 10 erg, presumably in a dense circumstellar environment as a result of the mass loss by a slow but dense stellar wind. This idea matches well with the aforementioned observations of the Crab, plus the richness of the He abundance (MacAlpine & Satterfield, 2008), an extreme brightness in the historical records (Sollerman et al., 2001; Tominaga et al., 2013; Moriya et al., 2014), and the observed nebular size (Yang & Chevalier, 2015). If this is the case, we should rather search for the shell much closer to the Crab.

The X-ray band is most suited to search for the thermal emission from a 10–10 K plasma expected from the shocked material forming a shell. In the past, telescopes with a high spatial resolution were used to set an upper limit on the thermal X-ray emission from the Crab (Mauche & Gorenstein, 1985; Predehl & Schmitt, 1995; Seward et al., 2006). A high contrast imaging is required to minimize the contamination by scattered X-rays by the telescope itself and the interstellar dust around the Crab. Still, the vicinity of the Crab is inaccessible with the imaging technique for the overwhelmingly bright and non-uniform flux of the PWN.

Here, we present the result of a spectroscopic search for the thermal plasma using the soft X-ray spectrometer (SXS) onboard the Hitomi satellite (Takahashi et al., 2016). The SXS is a non-dispersive high-resolution spectrometer, offering a high contrast spectroscopy to discriminate the thermal emission or absorption lines from the bright featureless spectrum of the PWN. This technique allows access to the Crab’s vicinity and is complementary to the existing imaging results.

The goals of this paper are (1) to derive a new upper limit with the spectroscopic technique for the X-ray emitting plasma, (2) to assemble the upper limits by various techniques evaluated under the same assumptions, and (3) to compare with the latest hydro-dynamic (HD) calculations to examine if any SN explosion and environment models are consistent with the X-ray plasma limits. We start with the observations and the data reduction of the SXS in § 2, and present the spectroscopic search results of both the absorption and emission features by the thermal plasma in § 3. In § 4, we derive the upper limits on the physical parameters of the SN and the SNR using our results presented here and existing result in the literature, and compare with our HD simulations to gain insight into the origin of SN 1054.

2 Observations and Data Reduction

2.1 Observations

The SXS is a high-resolution X-ray spectrometer based on X-ray micro-calorimetry (Kelley et al., 2016). The HgTe absorbers placed in a 66 array absorb individual X-ray photons collected by the X-ray telescope, and the temperature increase of the Si thermometer is read out as a change in its resistance. Because of the very low heat capacity of the sensor controlled at a low temperature of 50 mK, a high spectral resolution is achieved over a wide energy range. The SXS became the first X-ray micro-calorimeter to have made observations of astronomical sources in the orbit and proved its excellent performance despite its short lifetime.

The Crab was observed on 2016 March 25 from 12:35 to 18:01 UT with the SXS. This turned out to be the last data set collected before the tragic loss of the spacecraft on the next day. The observation was performed as a part of the calibration program, and we utilize the data to present scientific results in this paper.

Figure 1 shows the 3\farcm0 3\farcm0 field of view on top of a Chandra image. The scale corresponds to 1.9 pc at a distance of 2.2 kpc (Manchester et al., 2005). This covers a significant fraction of the observed elliptical nebula with a diameter of 2.94.4 pc (Hester, 2008). The SXS was still in the commissioning phase (Tsujimoto et al., 2016), and some instrumental setups were non-nominal. Among them, the gate valve status was most relevant for the result presented here. The valve was closed to keep the Dewar in a vacuum on the ground, which was planned to be opened when we confirmed the initial outgassing had ceased in the spacecraft. This observation was made before this operation. As a result, the attenuation by a 260 m Be window of the gate valve (Eckart et al., 2016) limited the SXS bandpass to above 2 keV, which would otherwise extend down to 0.1 keV.

Figure 1: Field of view of the SXS superposed on the Chandra ACIS image after correcting for the readout streaks (Mori et al., 2004). The 66 pixels are shown with the top left corner uncovered for the calibration pixel. The numbers indicate the live time fraction only for pixels less than 0.980. The astrometry of the SXS events can be displaced by 20\farcs6 at 1 when the star tracker is unavailable. The position of the pulsar (Lobanov et al., 2011) and the halo center (Seward et al., 2006) are respectively shown with the cross and the plus signs.

The instrument had reached the thermal equilibrium by the time of the observation (Fujimoto et al., 2016; Noda et al., 2016). The detector gain was very stable except for the passage of the South Atlantic anomaly. The previous recycle operation of the adiabatic demagnetization refrigerators was started well before the observation at 10:20 on March 24, and the entire observation was within its 48-hour hold time (Shirron et al., 2016). The energy resolution was 4.9 eV measured with the Fe calibration source at 5.9 keV for the full width at the half maximum (Porter et al., 2016; Kilbourne et al., 2016; Leutenegger et al., 2016). This superb resolution is not compromised by the extended nature of the Crab nebula for being a non-dispersive spectrometer.

The actual incoming flux measured with the SXS was equivalent to 0.3 Crab in the 2–12 keV band due to the extra attenuation by the gate valve. The net exposure time was 9.7 ks.

2.2 Data Reduction

We started with the cleaned event list produced by the pipeline process version 03.01.005.005 (Angelini et al., 2016). Throughout this paper, we used the HEASoft and CALDB release on 2016 December 22 for the Hitomi collaboration. Further screening against spurious events was applied based on the energy versus pulse rise time. The screening based on the time clustering of multiple events was not applied; it is intended to remove events hitting the out-of-pixel area, but a significant number of false positive detection is expected for high count rate observations like this.

Due to the high count rate, some pixels at the array center suffer dead time (figure 1; Ishisaki et al. (2016)). Still, the observing efficiency of 72% for the entire array is much higher than conventional CCD X-ray spectrometers. For example, Suzaku XIS (Koyama et al., 2007) requires a 1/4 window 0.1 s burst clocking mode to avoid pile-up for a 0.3 Crab source, and the efficiency is only 5%. Details of the dead time and pile-up corrections are described in a separate paper. We only mention here that these effects are much less serious for the SXS than CCDs primarily due to a much faster sampling rate of 12.5 kHz and a continuous readout.

The source spectrum was constructed in the 2–12 keV range at a resolution of 0.5 eV bin. Events not contaminated by other events close in time (graded as Hp or Mp; Kelley et al. (2016)) were used for a better energy resolution. All pixels were combined. The redistribution matrix function was generated by including the energy loss processes by escaping electrons and fluorescent X-rays. The half power diameter of the telescope is 1\farcm2 (Okajima et al., 2016). The SXS has only a limited imaging capability, and we do not attempt to perform a spatially-resolved spectroscopic study in this paper. The SXS does have a timing resolution to resolve the 34 ms pulse phase, but we do not attempt a phase-resolved study either as only a small gain in the contrast of thermal emission against the pulse emission is expected; the unpulsed emission of a 90% level of averaged count rate can be extracted at a compensation of 2/3 of the exposure time.

The total number of events in the 2–12 keV range is 7.610. The background spectrum, which is dominated by the non-X-ray background, was accumulated using the data when the telescope was pointed toward the Earth. The non-X-ray background is known to depend on the strength of the geomagnetic field strength at the position of the spacecraft within a factor of a few. The history of the geomagnetic cut-off rigidity during the Crab observation was taken into consideration to derive the background rate as 8.610 s in the 2–12 keV band. This is negligible with 10 of the source rate.

3 Analysis

To search for signatures of thermal plasma, we took two approaches. One is to add a thermal plasma emission model, or to multiply a thermal plasma absorption model, upon the best-fit continuum model with an assumed plasma temperature, which we call plasma search (§ 3.1). Here, we assume that the feature is dominant either as emission or absorption. The other is a blind search of emission or absorption lines, in which we test the significance of an addition or a subtraction of a line model upon the best-fit continuum model (§ 3.2). For the spectral fitting, we used the Xspec package version 12.9.0u (Arnaud, 1996). The statistical uncertainties are evaluated at 1 unless otherwise noted.

3.1 Plasma search

3.1.1 Fiducial model

We first constructed the spectral model for the entire energy band. The spectrum was fitted reasonably well with a single power-law model with an interstellar extinction, which we call the fiducial model. Hereafter, all the fitting was performed for unbinned spectra based on the C statistics (Cash, 1979). For the extinction model by cold matter, we used the tbabs model version 2.3.2222See http://pulsar.sternwarte.uni-erlangen.de/wilms/research/tbabs/ for details. (Wilms et al., 2000). We considered the extinction by interstellar gas, molecules, and dust grains with the parameters fixed at the default values of the model except for the total column density. The SXS is capable of resolving the fine structure of absorption edges, which is not included in the model except for O K, Ne K, and Fe L edges. This, however, does not affect the global fitting, as the depths of other edges are shallow for the Crab spectrum.

We calculated the effective area assuming a point-like source at the center of the SXS field. The nebula size is no larger than the point spread function. Figure 2 shows the best-fit model, while table 1 summarizes the best-fit parameters for the extinction column by cold matter (), the power-law photon index (), and the X-ray flux (). The ratio of the data to the model show some broad features, which are attributable to the inaccuracies of the calibration including the mirror Au M and L edge features, the gate valve transmission, the line spread function, ray-tracing modeling accuracies, etc (Okajima et al. in prep.). In this paper, therefore, we constrain ourselves to search for lines that are sufficiently narrow to decouple with these broad systematic uncertainties. This is possible only with high-resolution spectrometers.

Figure 2: Best-fit fiducial model to the background-subtracted spectra binned only for display purpose. The top panel shows the data with crosses and the best-fit model with solid lines. The bottom panel shows the ratio to the fit.
Parameterfootnotemark: Best-fit
10 cm 4.6 (4.1–5.0)
2.17 (2.16–2.17)
erg s cmfootnotemark: 1.722 (1.719–1.728) 10
Red-/d.o.f. 1.34/19996
footnotemark:

The errors indicate a 1 statistical uncertainty.

footnotemark:

The absorption-corrected flux at 2–8 keV.

Table 1: Best-fit parameters of the global fitting.

3.1.2 Plasma emission

For the thermal plasma emission, we assumed the optically-thin collisional ionization equilibrium (CIE) plasma model and two non-CIE deviations from it. All the calculations were based on the atomic database ATOMDB (Foster et al., 2012) version 3.0.7. We assumed the solar abundance (Wilms et al., 2000). This gives a conservative upper limit for plasma with a super-solar metalicity when they are searched using metallic lines.

First, we used the apec model (Smith et al., 2001) for the CIE plasma, in which the electron, ion, and ionization temperatures are the same. Neither the bulk motion nor the turbulence broadening was considered, but the thermal broadening was taken into account for the lines. For each varying electron temperature (table 2), we selected the strongest emission line in the 10 non-overlapping 1 keV ranges in the 2–12 keV band. For each selected line, we first fitted the 50 eV range around the line with a power-law model, then added the plasma emission model to set the upper limit of the volume emission measure () of the plasma. Both power-law and plasma emission models were attenuated by an interstellar extinction model of a column density fixed at the fiducial value (table 1). We expect some systematic uncertainty in the value due to incomplete calibration at low energies. The best-fit value in the fiducial model (table 2) tends to be higher than those in the literature (Kaastra et al., 2009; Weisskopf et al., 2010) by 10–30%. A 10% decrease in the value leads to 10% decrease of for the temperature 1 keV. The normalization of the plasma model was allowed to vary both in the positive and negative directions so as not to distort the significance distribution. The result for selected cases is shown in figure 3.

Deviation from the thermal equilibrium is seen in SNR plasmas (Borkowski et al., 2001; Vink, 2012), especially for young SNRs expanding in a low density environment. We considered two types of deviations. One is the non-equilibrium ionization using the nei model (Smith & Hughes, 2010). This code calculates the collisional ionization as a function of the ionization age (), and accounts for the difference between the ionization and electron temperatures. The electron temperature is assumed constant, which is reasonable considering that some SNRs show evidence for the collision-less instantaneous electron heating at the shock (Yamaguchi et al., 2014b). We took the same procedure with the CIE plasma for the values listed in table 2, and derived the upper limit of .

Another non-CIE deviation is that the electron and ion temperatures are different. More massive ions are expected to have a higher temperature than less massive ions and electrons, hence are more thermally broadened before reaching equilibrium. We derived the upper limit of for several values of the ion’s thermal velocity (table 2). In this modeling, the continuum fit was performed over an energy range of the smaller of the two: (3 or 50) eV centered at the line energy , so as to decouple the continuum and line fitting when is large.

Figure 3: 3 statistical upper limits of the volume emission measure () for the assumed electron temperature for selected parameters (table 2): (a) CIE, (b) broadened lines by (1.5, 3.0, and 6.0) 10 km s, and (c) non-equilibrium cases with 10.5, 11.5, and 12.5. The name of ions giving the strongest emission line for (a) at each temperature is shown at the top.
Par Unit Description Totalfootnotemark: Casesfootnotemark:
keV Electron temperature 21 0.1–10 (0.1 dex step)
footnotemark: s cm Ionization age 8 10.0–13.5 (0.5 step)
footnotemark: Thermal broadening of lines 5 0.001, 0.002, 0.005, 0.01, 0.02
footnotemark: Shell fraction 6 0.005, 0.01, 0.05, 0.083 (1/12), 0.10, 0.15
footnotemark:

The parameter is searched only for the plasma emission (§ 3.1.2).

footnotemark:

The ion spices has a velocity , thus has a temperature of /, in which is the mass of the ion. In the case of Si and Fe, the cases correspond to 12 MeV and 21 MeV.

footnotemark:

The value 1/12 is for the self-similar solution (Sedov, 1959), and 0.15 follows preceding work (Seward et al., 2006; Frail et al., 1995).

footnotemark:

The adopted parameters (cases) and the total number of cases (total) are shown.

Table 2: Investigated parameter space.

3.1.3 Plasma absorption

A similar procedure was taken for deriving the upper limit for the absorption column by a thermal plasma. We used the hotabs model (Kallman & Bautista, 2001) and only considered the CIE plasma. At each assumed electron temperature (table 2), we selected the strongest absorption line in the 10 non-overlapping 1 keV ranges in the 2–12 keV band. For each selected line, we first fitted the 50 eV range around the line with a power-law model, then multiplied the plasma absorption model to set the upper limit of the hydrogen-equivalent absorption column () by the plasma. The result is shown in figure 4.

Figure 4: 3 statistical upper limits of the hydrogen-equivalent extinction column () by the CIE plasma for the assumed electron temperature. The name of ions giving the strongest absorption line at each temperature is shown at the bottom

3.1.4 Example in the Fe K band

For the emission, the resultant upper limit of is less constrained for plasma with lower temperatures. At low temperatures, strong lines are at energies below 2 keV, in which the SXS has no sensitivity as the gate valve was not opened. For increasing temperatures above 0.5 keV, S-He, Ar-He, or Fe-He are used to set the limit. The most stringent limit is obtained at the maximum formation temperature (5 keV) of the Fe-He line. For the NEI plasma with a low ionization age (10 s cm), He-like Fe ions have not been formed yet, thus the limit is not stringent. Conversely, at an intermediate ionization age (10 s cm), Fe is not fully ionized yet, thus Fe-He can give a strong upper limit even for electron temperatures of 10 keV. At 10 s cm, the result is the same with the CIE plasma as expected.

Figure 5 shows a close-up view of the fitting around the Fe-He line for the case of the 3.16 keV electron temperature. Overlaid on the data, models are shown in addition to the best-fit power-law continuum model. Also shown is the expected result by a CCD spectrometer, with which the levels detectable easily with the SXS would be indistinguishable from the continuum emission. This demonstrates the power of an X-ray micro-calorimeter for weak features from extended sources. The expected energy shifts for a bulk velocity of 10 km s, or 22.4 eV, are shown. The data quality is quite similar in this range, thus the result is not significantly affected by a possible gain shift (1 eV; Hitomi Collaboration et al. (2016)) or a single bulk velocity shift.

Figure 5: Close-up view around the Fe-He resonance line. Over the unbinned spectrum (gray plus signs), several models are shown: the best-fit continuum model (black dashed), and the emission (solid) and absorption (dashed) by a 3.16 keV CIE plasma with 3 upper limits (blue) corresponding to  cm for emission and  cm for absorption. Ten times the absorption value is also shown with green (SXS) and purple (convolved with a Suzaku XIS response).

3.2 Blind search

We searched for emission or absorption line features at an arbitrary line energy in the 2–12 keV range. We made trials at 20,000 energies separated by 0.5 eV. The trials were repeated for a fixed line width corresponding to a velocity of 0, 20, 40, 80, 160, 320, 640, and 1280 km s. For each set of line energy and width, we fitted the spectrum with a power-law model locally in an energy range 3–20 on both sides of the trial energy . Here, the unit of the fitting range is determined as

(1)

in which is the 1 width of the Gaussian core of the detector response (Leutenegger et al., 2016). With this variable fitting range, we can test a wide range of line energy and width. After fixing the best-fit power-law model, we added a Gaussian model allowing both positive and negative amplitudes respectively for emission and absorption lines and refitted in the 0–20 on both sides. The detection significance was evaluated as

(2)

in which and are the best-fit and 1 statistical uncertainty of the line normalization in the unit of s cm, whereas and are those of the continuum intensity in the unit of s cm keV at the line energy.

Figure 6 shows the distribution of the significance. All are reasonably well fitted by a single Gaussian distribution. We tested several different choices of fitting ranges and confirmed that the overall result does not change. Above a 5 level (0.01 false positives expected for 20,000 trials) of the best-fit Gaussian distribution, no significant detection was found except for (1) several detections of absorption in the 2.0–2.2 keV energy range for a wide velocity range, and (2) a detection of absorption at 9.48 keV for 160 and 320 km s. The former is likely due to the inaccurate calibration of the Au M edges of the telescope. For the latter, no instrumental features or strong atomic transitions are known around this energy. However, we do not consider this to be robust as it escapes detection only by changing the fitting ranges.

Figure 6: Distribution of significance (eqn 2) for different assumed velocities in different colors. The distribution is fitted by a single Gaussian model, and its best-fit parameters are shown in the legend as (center/width). The vertical dotted lines indicate the 5 level of the best-fit Gaussian distribution.

The equivalent width, was derived for every set of the line energy and width along with their 3 statistical uncertainty (figure 7). The 3 limit of EW at 6.4 keV is 2 eV. We would expect the Fe fluorescence line with in which for the Crab’s power-law spectrum (Krolik & Kallman, 1987). and are, respectively, the subtended angle and the H-equivalent column of the fluorescing matter around the incident emission. Assuming and  cm, which is the measured value in the line of sight inclusive of the ISM (Mori et al., 2004), the expected EW is consistent with the upper limit by the SXS.

Figure 7: 3 range of the equivalent width for different assumed velocities. The curves are obtained by convolving the fitting result at each energy bin with a low pass filter. A structure at 11.9 keV is due to the Au L absorption edge by the telescope.

4 Discussion

In § 4.1, we convert the upper limit of or with the SXS into that of the plasma density () by making several assumptions. In § 4.2, we re-evaluate the data by other methods in the literature under the same assumptions to assemble the most stringent upper limit of for various ranges of the parameters. In § 4.3, we perform a HD calculation for some SN models and verify that the searched parameter ranges are reasonable. In § 4.4, we compare the HD result with observed limits.

4.1 Constraints on the plasma density with SXS

For converting the upper limits of and of the thermal plasma into that of the X-ray emitting plasma density (), we assume the plasma is uniform in a spherically symmetric shell in a range of to from the center. We assumed several shell fraction () values (table 2). For simplicity, the electron and ion densities are the same, and all ions are hydrogen. This gives a conservative upper limit for the plasma mass.

We first use the upper limit of the plasma emission. The density is , in which is the observed emitting volume. Some selected cases are shown in figure 8 (thick solid and dashed curves). If the SXS square field of view with 3\farcm0 covers the entire shell at  1\farcm3, . If the field is entirely contained in the shell at  2\farcm1, should be replaced with , in which is the distance to the source. These approximations at the two ends make a smooth transition.

Figure 8: Upper limits to the plasma density for several selected electron temperatures of a CIE (solid) and an NEI with  s cm (dotted) plasmas as a function of the assumed shell radius for the SXS (thick) and ACIS (thin; Seward et al. (2006)) when the shell fraction is . The observed limits move vertically when the shell fraction is changed by the scaling shown in the figure. The effective area for the projected shell distribution is shown with green points with statistical uncertainties by the ray-tracing simulations, which is smoothed (green dashes) by the Savitzky & Golay (1964) method to use for the correction. The star marks are the expected limit with off-source pointing with the SXS at 2.6 and 4.1 pc for the CIE of different temperatures.

Here, we made a correction for the reduced effective area for the extended structure of the shell. As increases within the SXS field of view, the effective area averaged over the view decreases as more photons are close to the field edges. This effect is small in the case of the Crab because the central pixels suffer dead time due to the high count rate (figure 1). In fact, a slightly extended structure up to 1\farcm2 has a larger effective area than a point-like distribution. As increase beyond the field, the emission within the field becomes closer to a flat distribution, and the reduction of the effective area levels off (figure 8; green data and dashed curve).

Next, we convert the upper limits by the extinction column to the density with , which is shown in figure 9 (thick lines). We assume that the absorption feature is superposed on a point-like continuum source, thus no correction was made for the extended structure.

Figure 9: Upper limits to the plasma density for several selected electron temperatures of a CIE plasma as a function of the assumed shell radius for the SXS (thick) and RGS (thin; Kaastra et al. (2009)) when the shell fraction is . The observed limits move vertically when the shell fraction is changed by the scaling shown in the figure. Also shown is the upper limit by a radio dispersion measure (DM) of the Crab pulsar (Lundgren et al., 1995).

4.2 Results with other techniques

We compare the results with the previous work using three different techniques. First, Seward et al. (2006) used the Advanced CCD Imaging Spectrometer (ACIS; Garmire et al. (2003)) onboard the Chandra X-ray Observatory (Weisskopf et al., 2002) with an unprecedented imaging resolution, and derived the upper limit of the thermal emission assuming that it would be detectable if it has a 0.1 times surface brightness of the observed halo emission attributable to the dust scattering. We re-evaluated their raw data (their figure 5) under the same assumptions with SXS (figure 8; thin solid and dashed curves). No ACIS limit was obtained below \arcmin due to the extreme brightness of the PWN. Beyond \arcmin, at which there is no ACIS measurement, we used the upper limit at 18\arcmin. For the ACIS limits, a more stringent limit is obtained for the NEI case with a low ionization age (10 s cm) than the CIE case with the same temperature. This is because the Fe L series lines are enhanced for such NEI plasmas and the ACIS is sensitive also at 2 keV unlike the SXS with the gate valve closed.

Second, Kaastra et al. (2009) presented the Crab spectrum using the Reflecting Grating Spectrometer (RGS; den Herder et al. (2001)) onboard the XMM-Newton Observatory (Jansen et al., 2001) observatory. Upon the non-thermal emission of the PWN, they reported a detection of the absorption feature by the O-He and O-Ly lines respectively at 0.58 and 0.65 keV with a similar equivalent width of 0.2 eV assuming that the lines are narrow. The former was also confirmed in the Chandra Low Energy Transmission Grating data. However, these absorption lines are often seen in the spectra of Galactic X-ray binaries (e.g., Yao & Wang (2006)), which is attributed to the hot gas in the interstellar medium with a temperature of a few MK. Adopting the value by Sakai et al. (2014), the expected column density by such a gas to the Crab is 810 cm, which is non-negligible. We therefore consider that the values measured with RGS are an upper limit for the plasma around the Crab. Using the same assumptions with SXS, we re-evaluated the RGS limit (thin lines in figure 9).

Third, the dispersion measure from the Crab pulsar reflects the column density of ionized gas along the line of sight. This includes not only the undetected thermal plasma around the Crab but also the hot and warm interstellar gas. Lundgren et al. (1995) derived a measure 1.810 cm, which converts to another density limit (dashed line in figure 9).

We now have the upper limit on for several sets of , , and by assembling the lowest values among various methods (re)-evaluated under the same assumptions. We convert the limit to that of the total X-ray emitting mass , where is the proton mass and is the total emitting volume for an assumed shell size and fraction. The resultant limit is shown in Figure 10. The most stringent limit is given by the emission search either by ACIS or SXS. The SXS result complements the ACIS result at  pc, and the two give an upper limit of 1 for the X-ray emitting plasma at any shell radius. The exception is for the low plasma temperature below 1 keV, for which the SXS with the closed gate valve yields a less constraining limit.

Figure 10: Upper limit of the total plasma mass when the shell has a size for several electron temperatures of the CIE (solid) and NEI with  s cm (dotted) plasmas. is assumed. The observed limits move vertically when the shell fraction is changed by the scaling shown in the figure. The position of (, ) is shown for the models in table 3 with the stars, and their direction of change when is changed by a factor of 10 or 0.1 (dotted-and-dashed green lines from the stars).

4.3 HD calculation

We performed a HD calculation to verify that the searched parameter ranges (table 2) are reasonable and to confirm if there are any SN models consistent with the observed limit. We used the CR-hydro-NEI code (Lee et al. (2014) and references therein), which calculates time-dependent, non-equilibrium plasma in one dimension. At the forward shock, the kinetic energy is thermalized independently for each species, thus the temperature is proportional to the mass of the species. The plasma is then thermally relaxed by the Coulomb interaction. No collissionless shocks are included. Energy loss by radiation is included, while that by cosmic rays is omitted.

We considered two SN explosion models under two circumstellar environments (table 3) as representatives. The former two are (a) the Fe-core collapse SN by a red super-giant progenitor with the initial explosion energy  erg and the ejecta mass   (Patnaude et al., 2015), and (b) the electron capture (EC) SN by a super AGB progenitor with  erg and   (Moriya et al., 2014). The latter two are (1) the uniform density  cm and (2) the density profile by the progenitor wind: , in which the mass loss rate  yr and the wind velocity  km s (Moriya et al., 2014). In the wind density parameter (Chugai & Danziger, 1994),  g cm.

The 22 models are labeled as (a-1) Fe-I, (a-2) Fe-w, (b-1) EC-I, and (b-2) EC-w. For Fe-I and EC-I models, we also calculated an elevated ISM density of  cm (respectively labeled as Fe-I and EC-I). For all these models, we assumed the power () of the unshocked ejecta density as a function of velocity to be 9 (Fransson et al., 1996). Only for the model EC-w, we calculated with to see the effect of this parameter (labeled as EC-w).

Label Fe-I Fe-I Fe-w EC-I EC-I EC-w EC-w
(SN setup)
SN explosion Fe Fe Fe EC EC EC EC
(10 erg) 1.21 1.21 1.21 0.15 0.15 0.15 0.15
() 12.1 12.1 12.1 4.36 4.36 4.36 4.36
9 9 9 9 9 9 7
Environment ISM ISM wind ISM ISM wind wind
(cm) 0.1 1.0 0.1 1.0
(10 g cm) 3.2 3.2 3.2
(SNR outcome)
(pc) 4.6 3.6 4.3 2.9 2.2 2.3 2.6
(pc) 4.1 3.2 3.5 2.6 2.0 1.9 2.0
(pc) 3.8 2.9 3.3 2.4 1.8 1.8 1.9
(10 km s) 3.1 2.4 3.7 2.0 1.5 2.0 2.1
footnotemark: (10 km s) 1.4 1.2 0.51 0.88 0.68 0.29 0.39
() 1.4 6.6 2.0 0.35 1.6 1.1 1.2
() 1.8 7.0 4.1 0.42 2.2 2.2 1.3
—— derived values ——
0.07 0.07 0.04 0.06 0.09 0.04 0.07
(keV) 9.4 5.7 13 3.8 2.2 4.0 4.1
() 10 5.1 8.0 3.9 2.2 2.2 3.0
—— absorbed X-ray flux weighted average ——
(keV) 1.0 1.6 0.51 0.71 0.95 0.74 0.51
(keV) 130 26 50 57 4.0 62 90
(10 cm s) 0.21 1.5 9.9 0.22 1.59 11.8 10.2
() 0.67 5.0 2.0 0.14 1.2 0.81 1.1
footnotemark:

Velocity with respect to the ejecta.

Table 3: Result of HD calculation.

Table 3 summarizes the SN setup stated above and the SNR outcome at an age of 962 yr, which includes the radius of the forward shock (FS), contact discontinuity (CD), and reverse shock (RS) (, , and ), the velocity of the forward and reverse shocks ( and ), the mass between CD and FS () and that between RS and CD (). The two masses represent the shocked ISM and ejecta, respectively. The radius is close to the observed size of the optical photo-ionized nebula, and the radii and velocities match reasonably well with analytical approaches (Chevalier, 1982; Truelove & McKee, 1999) within 10%, which validates our calculation. The RS radius is larger than the X-ray emitting synchrotron nebula, which justifies that our calculation does not include the interaction with it.

From these, we calculated as a proxy for the shell fraction, as a proxy for the electron temperature after Coulomb relaxation, in which is the mean molecular weight, and the unshocked ejecta mass . We also derived the average of the electron and Fe temperatures ( and ) and the ionization age () weighted over the absorbed X-ray flux. The X-ray emitting mass () was estimated by integrating the mass with a temperature in excess of .

The searched ranges of all parameters (table 2) encompass the HD result for all models. The electron temperature is expected between and ; the former is the highest for thermalizing all the kinetic energy instantaneously, while the latter is the lowest for starting the Coulomb relaxation without collision-less heating. The averaged Fe temperature is sufficiently low to consider that the line is relatively narrow; the thermal broadening by this is 32 eV at 6.7 keV for =130 keV. The ionization age () ranges over two orders from 10 to 10 cm s depending on the pre-explosion environment, where the wind density cases result in higher values than the ISM density cases.

4.4 Comparison with observed limits

Finally, we compare the HD results with the observation in figure 10. For the radius and the X-ray plasma mass, we plotted (, ) in table 3. The shell size by the models () is larger than 1.3 pc, where we have a stringent limit on with the observations. The HD results depend on the choice of the parameters in the SN setup (, , or , and ; table 3). We can estimate in which direction the model points move in the plot when these parameters are changed.

First, the two parameters and are known to be correlated in type II SNe. Our two SN models are in line with the relation by Pejcha & Prieto (2015). Therefore, the model points move roughly in the direction of the lines connecting the EC-I and Fe-I models, or the EC-w and Fe-w models. For a fixed explosion energy of 1.2110 erg for our Fe model, a plausible range of is 12–32  (Pejcha & Prieto, 2015), thus our model is close to the lower bound. Second, for , the points move in parallel with the lines connecting Fe-I and Fe-I or EC-I and EC-I. This should be the same for in the wind environment case. Third, for , there is little difference between the result of the model Fe-w and Fe-w, so we consider that this parameter does not affect the result very much. In terms of the comparison with the observation limit, or is the most important factor.

Although the small observed mass of the Crab is argued to rule out an Fe core collapse SN for its origin (Seward et al., 2006), we consider that this does not simply hold. Our models illustrate that such a small mass can be reproduced if an Fe core collapse SN explosion takes place in a sufficiently low density environment with the ISM density 0.03 cm (Fe-I) or the wind density parameter  g cm (Fe-w). In such a case, a large fraction of the ejecta mass is unshocked (table 3) and escapes from detection. Some of the unshocked ejecta may be visible when they are photo-ionized by the emission from the PWN to a 10 K gas (Fesen et al., 1997) or a 10 K gas (Sollerman et al., 2000).

We argue that both the Fe and EC models still hold to be compatible with the observed mass limits. In either case, it is strongly preferred that the pre-explosion environment is low in density; i.e.,  cm (EC-I) or  cm (Fe-I) for the ISM environment or  g cm for the wind environment (both Fe-w and EC-w). For the latter, a large value (e.g.,  g cm; Smith (2013)), which is an idea to explain the initial brightness of SN 1054, is not favored. In fact, such a low density environment is suggested by observations. At the position of the Crab, which is off-plane in the anti-Galactic center direction, the ISM density is 0.3 cm by a Galactic model (Ferrière, 1998). Wallace et al. (1999) further claimed the presence of a bubble around the Crab based on an H I mapping with a density lower than the surroundings. Our result suggests that SN 1054 took place in such a low environment and the wind environment by its progenitor of a low wind density value.

5 Conclusion

We utilized the SXS calibration data of the Crab nebula in 2–12 keV to set an upper limit to the thermal plasma density by spectroscopically searching for emission or absorption features in the Crab spectrum. No significant emission or absorption features were found in both the plasma and the blind searches.

Along with the data in the literature, we evaluated the result under the same assumptions to derive the X-ray plasma mass limit to be for a wide range of assumed shell radii () and plasma temperatures (). The SXS sets a new limit in  1.3 pc for  keV. We also performed HD simulations of the Crab SNR for two SN explosion models under two pre-explosion environments. Both SN models are compatible with the observed limits when the pre-explosion environment has a low density of 0.03 cm (Fe model) or 0.1 cm (EC model) for the uniform density, or 10 g cm ( yr for  20 km s) for the wind density parameter in the wind environment.

A low energy explosion is favored based on the abundance, initial light curve, and nebular size studies (MacAlpine & Satterfield, 2008; Moriya et al., 2014; Yang & Chevalier, 2015). We believe that a positive detection of thermal plasma, in particular with lines, is key to distinguishing the Fe and EC models. It is worth noting that the observed limit is close to the model predictions. We now know the high potential of a spectroscopic search with the SXS, and may expect a detection of the thermal feature by placing the SXS field center at several offset positions. With a 10 ks snapshot at four different positions at the radius of EC-I and Fe-I models (respectively 2.6 and 4.1 pc), an upper limit lower than that with ACIS by a factor of a few is expected (figure 8).

This was exactly what was planned next. If it were not for the loss of the spacecraft estimated to have happened at 1:42 UT on 2016 March 26, a series of the offset Crab observations should have started 8 hours later for calibration purposes, which should have been followed by the gate valve open to allow access down to 0.1 keV. The 8 hours now turned to be many years, but we should be back as early as possible.

Author contributions

M. Tsujimoto led this study in data analysis and writing drafts. He also contributed to the SXS hardware design, fabrication, integration and tests, launch campaign, in-orbit operation, and calibration. S.-H. Lee performed the hydro calculations and its interpretations for this paper. K. Mori and H. Yamaguchi contributed to discussion on SNRs. They also made hardware and software contributions to the Hitomi satellite. N. Tominaga and T. J. Moriya gave critical comments on SNe. T. Sato worked for the telescope response for the data analysis and calibration. C. de Vries led the filter wheel of the SXS, which gave the only pixel-to-pixel gain reference of this spectrometer in the orbit. R. Iizuka contributed to the testing and calibration of the telescope, and the operation of the SXS. A. R. Foster and T. Kallman helped with the plasma models. M. Ishida, R. F. Mushotzky, A. Bamba, R. Petre, B. J. Williams, S. Safi-Harb, A. C. Fabian, C. Pinto, L. C. Gallo, E. M. Cackett, J. Kaastra, M. Ozaki, J. P. Hughes, and D. McCammon improved the draft.

{ack}

We appreciate all people contributed to the SXS, which made this work possible. We also thank Toru Misawa in Shinshu University for discussing the C IV feature.

We thank the support from the JSPS Core-to-Core Program. We acknowledge all the JAXA members who have contributed to the ASTRO-H (Hitomi) project. All U.S. members gratefully acknowledge support through the NASA Science Mission Directorate. Stanford and SLAC members acknowledge support via DoE contract to SLAC National Accelerator Laboratory DE-AC3-76SF00515. Part of this work was performed under the auspices of the U.S. DoE by LLNL under Contract DE-AC52-07NA27344. Support from the European Space Agency is gratefully acknowledged. French members acknowledge support from CNES, the Centre National d’Études Spatiales. SRON is supported by NWO, the Netherlands Organization for Scientific Research. Swiss team acknowledges support of the Swiss Secretariat for Education, Research and Innovation (SERI). The Canadian Space Agency is acknowledged for the support of Canadian members. We acknowledge support from JSPS/MEXT KAKENHI grant numbers 15H00773, 15H00785, 15H02090, 15H03639, 15H05438, 15K05107, 15K17610, 15K17657, 16H00949, 16H06342, 16K05295, 16K05300, 16K13787, 16K17672, 16K17673, 21659292, 23340055, 23340071, 23540280, 24105007, 24540232, 25105516, 25109004, 25247028, 25287042, 25400236, 25800119, 26109506, 26220703, 26400228, 26610047, 26800102, JP15H02070, JP15H03641, JP15H03642, JP15H03642, JP15H06896, JP16H03983, JP16K05296, JP16K05309, JP16K17667, and 16K05296. The following NASA grants are acknowledged: NNX15AC76G, NNX15AE16G, NNX15AK71G, NNX15AU54G, NNX15AW94G, and NNG15PP48P to Eureka Scientific. H. Akamatsu acknowledges support of NWO via Veni grant. C. Done acknowledges STFC funding under grant ST/L00075X/1. A. Fabian and C. Pinto acknowledge ERC Advanced Grant 340442. P. Gandhi acknowledges JAXA International Top Young Fellowship and UK Science and Technology Funding Council (STFC) grant ST/J003697/2. Y. Ichinohe, K. Nobukawa, H. Seta, and T. Sato are supported by the Research Fellow of JSPS for Young Scientists. N. Kawai is supported by the Grant-in-Aid for Scientific Research on Innovative Areas “New Developments in Astrophysics Through Multi-Messenger Observations of Gravitational Wave Sources”. S. Kitamoto is partially supported by the MEXT Supported Program for the Strategic Research Foundation at Private Universities, 2014-2018. B. McNamara and S. Safi-Harb acknowledge support from NSERC. T. Dotani, T. Takahashi, T. Tamagawa, M. Tsujimoto and Y. Uchiyama acknowledge support from the Grant-in-Aid for Scientific Research on Innovative Areas “Nuclear Matter in Neutron Stars Investigated by Experiments and Astronomical Observations”. N. Werner is supported by the Lendület LP2016-11 grant from the Hungarian Academy of Sciences. D. Wilkins is supported by NASA through Einstein Fellowship grant number PF6-170160, awarded by the Chandra X-ray Center, operated by the Smithsonian Astrophysical Observatory for NASA under contract NAS8-03060.

We thank contributions by many companies, including in particular, NEC, Mitsubishi Heavy Industries, Sumitomo Heavy Industries, and Japan Aviation Electronics Industry. Finally, we acknowledge strong support from the following engineers. JAXA/ISAS: Chris Baluta, Nobutaka Bando, Atsushi Harayama, Kazuyuki Hirose, Kosei Ishimura, Naoko Iwata, Taro Kawano, Shigeo Kawasaki, Kenji Minesugi, Chikara Natsukari, Hiroyuki Ogawa, Mina Ogawa, Masayuki Ohta, Tsuyoshi Okazaki, Shin-ichiro Sakai, Yasuko Shibano, Maki Shida, Takanobu Shimada, Atsushi Wada, Takahiro Yamada; JAXA/TKSC: Atsushi Okamoto, Yoichi Sato, Keisuke Shinozaki, Hiroyuki Sugita; Chubu U: Yoshiharu Namba; Ehime U: Keiji Ogi; Kochi U of Technology: Tatsuro Kosaka; Miyazaki U: Yusuke Nishioka; Nagoya U: Housei Nagano; NASA/GSFC: Thomas Bialas, Kevin Boyce, Edgar Canavan, Michael DiPirro, Mark Kimball, Candace Masters, Daniel Mcguinness, Joseph Miko, Theodore Muench, James Pontius, Peter Shirron, Cynthia Simmons, Gary Sneiderman, Tomomi Watanabe; ADNET Systems: Michael Witthoeft, Kristin Rutkowski, Robert S. Hill, Joseph Eggen; Wyle Information Systems: Andrew Sargent, Michael Dutka; Noqsi Aerospace Ltd: John Doty; Stanford U/KIPAC: Makoto Asai, Kirk Gilmore; ESA (Netherlands): Chris Jewell; SRON: Daniel Haas, Martin Frericks, Philippe Laubert, Paul Lowes; U of Geneva: Philipp Azzarello; CSA: Alex Koujelev, Franco Moroso.

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