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Spectral Evolution of GRB060904A Observed with Swift and Suzaku- Possibility of Inefficient Electron Acceleration

Daisuke Yonetoku14NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USASachiko Tanabe1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]Toshio Murakami1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]N. Emura1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]Yuka Aoyama1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]T. Kidamura1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]H. Kodaira1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]Yoshiki Kodama1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]R. Kozaka1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]T. Nashimoto1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]S. Okuno1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]Satoshi Yokota1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]S. Yoshinari1Department of Physics, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192 [email protected]Keiichi Abe2Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570K. Onda2Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570M. Tashiro2Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570Y. Urata2Department of Physics, Saitama University, 255 Shimo-Okubo, Sakura, Saitama 338-8570Y. E. Nakagawa3Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8551Satoshi Sugita3Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8551K. Yamaoka3Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8551A. Yoshida3Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Sagamihara, Kanagawa 229-8551Takuto Ishimura4Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551N. Kawai4Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551Takashi Shimokawabe4Department of Physics, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551Kenzo Kinugasa5Gunma Astronomical Observatory, 6860-86 Nakayama, Takayama-mura, Agatsuma-gun, Gunma 377-0702Takayoshi Kohmura6Physics Department, Kogakuin University, 2665-1 Nakano, Hachioji, Tokyo 192-0015Kaori Kubota7Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502Kei Sugiyasu7Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502Yoshihiro Ueda7Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502Kensuke Masui8Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510Kazuhiro Nakazawa8Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510Tadayuki Takahashi8Institute of Space and Astronautical Science, JAXA, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510S. Maeno9University of Miyazaki, 1-1 Gakuen Kibanadai-Nishi, Miyazaki, Miyazaki 889-2192E. Sonoda9University of Miyazaki, 1-1 Gakuen Kibanadai-Nishi, Miyazaki, Miyazaki 889-2192M. Yamauchi9University of Miyazaki, 1-1 Gakuen Kibanadai-Nishi, Miyazaki, Miyazaki 889-2192Makoto Kuwahara10RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198Toru Tamagawa10RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198Daisuke Matsuura12Department of Physics, Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043Motoko SuzukiA. P. Beardmore14NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USAN. Gehrels14NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USAJ. A. Nousek15Pennsylvania State University, University Park, PA 16802, USA

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Annotatsiya

We observed an X-ray afterglow of GRB060904A with the Swift and Suzaku satellites. We found rapid spectral softening during both the prompt tail phase and the decline phase of an X-ray flare in the Swift/BAT and Swift/XRT data. The observed spectra were fit by power-law photon indices which rapidly changed from $\Gamma =$ 1.51$^{+0.04}_{-0.03}$ to $\Gamma =$ 5.30$^{+0.69}_{-0.59}$ within a few hundred seconds. This is one of the steepest X-ray spectra ever observed, making it quite difficult to explain by simple electron-acceleration and synchrotron-radiation processes. We then applied an alternative broken power-law with exponential cutoff (BPEC) model. It is valid to consider the exponential shape is equivalent to a synchrotron cutoff. Since the spectral cutoff appears in the soft X-ray band, we conclude that the electron acceleration must be inefficient in the internal shocks of GRB060904A. These cutoff spectra suddenly disappeared at the end of the prompt tail. After that, typical afterglow spectra with $\Gamma =$ 2.0 have been continuously and preciously monitored by both Swift/XRT and Suzaku/XIS. We could successfully trace the temporal history of two characteristic break energies that have a time dependence of $\propto t^{-3}$-$t^{-4}$, while the following afterglow spectra are quite stable. This fact indicates that the emitting material of the prompt tail is due to completely different dynamics from the shallow decay component. Therefore, the emission sites of two distinct phenomena obviously differ from each other.

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DOI: 10.1093/pasj/60.sp1.s351

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