Detection of a spectral break in the extra hard component of GRB 090926A

M. Ackermann, M. Ajello, K. Asano, M. Axelsson, L. Baldini, J. Ballet, G. Barbiellini, M.G. Baring, D. Bastieri, K. Bechtol, R. Bellazzini, B. Berenji, P.N. Bhat, E. Bissaldi, R. Blandford, E. Bonamente, A.W. Borgland, A. Bouvier, J. Bregeon, A. BrezM.S. Briggs, M. Brigida, P. Bruel, R. Buehler, S. Buson, G.A. Caliandro, R.A. Cameron, P.A. Caraveo, S. Carrigan, J.M. Casandjian, C. Cecchi, O. Celik, V. Chaplin, E. Charles, A. Chekhtman, J. Chiang, S. Ciprini, R. Claus, J. Cohen-Tanugi, V. Connaughton, J. Conrad, S. Cutini, C. Dermer, A. de Angelis, F. de Palma, B. Dingus, E. do Couto e Silva, P.S. Drell, R. Dubois, C. Favuzzi, S. Fegan, E.C. Ferrara, W.B. Focke, M. Frailis, Y. Fukazawa, S. Funk, P. Fusco, F. Gargano, D. Gasparrini, N. Gehrels, N. Giglietto, N. Giordano, N. Vilchez

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    Abstract

    We report on the observation of the bright, long gamma-ray burst,GRB 090926A, by the Gamma-ray Burst Monitor (GBM) and Large Area Tele-scope (LAT) instruments on board the Fermi Gamma-ray Space Telescope. GRB 090926A shares several features with other bright LAT bursts. In particular, it clearly shows a short spike in the light curve that is present in all detectors that see the burst, and this in turn suggests that there is a common region of emission across the entire Fermi energy range. In addition, while a separate high-energy power-law component has already been observed in other GRBs, here we report for the first time the detection with good significance of a high-energy spectral break (or cutoff) in this power-law component around 1.4 GeV in the time-integrated spectrum. If the spectral break is caused by opacity to electron-positron pair production within the source, then this observation allows us to compute the bulk Lorentz factor for the outflow, rather than a lower limit.
    Original languageEnglish
    JournalThe Astrophysical Journal
    Volume729
    Issue number2
    DOIs
    Publication statusPublished - 2011

    Keywords

    • gamma rays: bursts

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