DETECTION OF A SPECTRAL BREAK IN THE EXTRA HARD COMPONENT OF GRB 090926A

Ackermann, M; Ajello, M; Asano, K; Axelsson, M; Baldini, Luca; Ballet, J; Barbiellini, G; Baring, Mg; Bastieri, D; Bechtol, K; Bellazzini, R; Berenji, B; Bhat, Pn; Bissaldi, E; Blandford, Rd; Bonamente, E; Borgland, Aw; Bouvier, A; Bregeon, J; Brez, A; Briggs, Ms; Brigida, M; Bruel, P; Buehler, R; Buson, S; Caliandro, Ga; Cameron, Ra; Caraveo, Pa; Carrigan, S; Casandjian, Jm; Cecchi, C; Celik, O; Chaplin, V; Charles, E; Chekhtman, A; Chiang, J; Ciprini, S; Claus, R; Cohen Tanugi, J; Connaughton, V; Conrad, J; Cutini, S; Dermer, Cd; De Angelis, A; De Palma, F; Dingus, Bl; Edce, Silva; Drell, Ps; Dubois, R; Favuzzi, C; Fegan, Sj; Ferrara, Ec; Focke, Wb; Frailis, M; Fukazawa, Y; Funk, S; Fusco, P; Gargano, F; Gasparrini, D; Gehrels, N; Germani, S; Giglietto, N; Giordano, F; Giroletti, M; Glanzman, T; Godfrey, G; Goldstein, A; Granot, J; Greiner, J; Grenier, Ia; Grove, Je; Guiriec, S; Hadasch, D; Hanabata, Y; Harding, Ak; Hayashi, K; Hayashida, M; Hays, E; Horan, D; Hughes, Re; Itoh, R; Johannesson, G; Johnson, As; Johnson, Wn; Kamae, T; Katagiri, H; Kataoka, J; Kippen, Rm; Knodlseder, J; Kocevski, D; Kouveliotou, C; Kuss, M; Lande, J; Latronico, L; Lee, Sh; Garde, Ml; Longo, F; Loparco, F; Lovellette, Mn; Lubrano, P; Makeev, A; Mazziotta, Mn; Mcbreen, S; Mcenery, Je; Mcglynn, S; Meegan, C; Mehault, J; Meszaros, P; Michelson, Pf; Mizuno, T; Monte, C; Monzani, Me; Moretti, E; Morselli, A; Moskalenko, Iv; Murgia, S; Nakajima, H; Nakamori, T; Naumann Godo, M; Nishino, S; Nolan, Pl; Norris, Jp; Nuss, E; Ohno, M; Ohsugi, T; Okumura, A; Omodei, N; Orlando, E; Ormes, Jf; Ozaki, M; Paciesas, Ws; Paneque, D; Panetta, Jh; Parent, D; Pelassa, V; Pepe, M; Pesce Rollins, M; Petrosian, V; Piron, F; Porter, Ta; Preece, R; Racusin, Jl; Raino, S; Rando, R; Rau, A; Razzano, Massimiliano; Razzaque, S; Reimer, A; Reimer, O; Reposeur, T; Reyes, Lc; Ripken, J; Ritz, S; Roth, M; Ryde, F; Hfw, Sadrozinski; Sander, A; Scargle, Jd; Strickman, Ms; Suson, Dj; Tajima, H; Takahashi, H; Tanaka, T; Tanaka, Y; Thayer, Jb; Thayer, Jg; Tibaldo, L; Tierney, D; Toma, K; Torres, Df; Tosti, G; Tramacere, A; Uchiyama, Y; Uehara, T; Usher, Tl; Vandenbroucke, J; van der Horst AJ,; Vasileiou, V; Vilchez, N; Vitale, V; von Kienlin, A; Waite, Ap; Wang, P; Wilson Hodge, C; Winer, Bl; Wood, Ks; Xf, Wu; Yamazaki, R; Yang, Z; Ylinen, T; Ziegler, M.

doi:10.1088/0004-637X/729/2/114

We report on the observation of the bright, long gamma-ray burst, GRB 090926A, by the Gamma-ray Burst Monitor and Large Area Telescope (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 gamma-ray bursts, 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.