title: On-ground detection of an electron-positron annihilation line from thunderclouds.
abstract: Thunderclouds can produce bremsstrahlung gamma-ray emission, and sometimes even positrons. At 00:27:00 (UT) on 13 January 2012, an intense burst of gamma rays from a thundercloud was detected by the GROWTH experiment, located in Japan, facing the Sea of Japan. The event started with a sharp gamma-ray flash with a duration of <300 ms coincident with an intracloud discharge, followed by a decaying longer gamma-ray emission lasting for ∼60 s. The spectrum of this prolonged emission reached ∼10 MeV, and contained a distinct line emission at 508±3(stat.)±5(sys.) keV, to be identified with an electron-positron annihilation line. The line was narrow within the instrumental energy resolution (∼80keV), and contained 520±50 photons which amounted to ∼10% of the total signal photons of 5340±190 detected over 0.1-10 MeV. As a result, the line equivalent width reached 280±40 keV, which implies a nontrivial result. The result suggests that a downward positron beam produced both the continuum and the line photons.
authors.affiliation: Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: D
authors.initials: D
authors.lastname: Umemoto
authors.affiliation: High Energy Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.|Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki 319-1195, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Tsuchiya
authors.affiliation: High Energy Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.|NASA Goddard Space Flight Center, Astrophysics Science Division, Code 662, Greenbelt, Maryland 20771, USA.
authors.forename: T
authors.initials: T
authors.lastname: Enoto
authors.affiliation: Department of Physics, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-0397, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Yamada
authors.affiliation: High Energy Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Yuasa
authors.affiliation: Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science, JAXA, Sagamihara, Kanagawa 252-5210, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Kawaharada
authors.affiliation: Department of Physical Sciences, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Kitaguchi
authors.affiliation: Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Nakazawa
authors.affiliation: Department of Space Astronomy and Astrophysics, Institute of Space and Astronautical Science, JAXA, Sagamihara, Kanagawa 252-5210, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Kokubun
authors.affiliation: High Energy Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.
authors.forename: H
authors.initials: H
authors.lastname: Kato
authors.affiliation: High Energy Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Okano
authors.affiliation: High Energy Astrophysics Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.
authors.forename: T
authors.initials: T
authors.lastname: Tamagawa
authors.affiliation: Department of Physics, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.|MAXI Team, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0193, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Makishima