Detection of a Spectral Break in the Extra Hard Component of GRB 090926A

Ackermann, M.; Ajello, M.; Asano, K.; Axelsson, M.; Baldini, L.; Ballet, J.; Barbiellini Amidei, Guido; Baring, M. G.; Bastieri, D.; Bechtol, K.; Bellazzini, R.; Berenji, B.; Bhat, P. N.; Bissaldi, Elisabetta; Blandford, R. D.; Bonamente, E.; Borgland, A. W.; Bouvier, A.; Bregeon, J.; Brez, A.; Briggs, M. S.; Brigida, M.; Bruel, P.; Buehler, R.; Buson, S.; Caliandro, G. A.; Cameron, R. A.; Caraveo, P. A.; Carrigan, S.; Casandjian, J. M.; Cecchi, C.; Celik, .; Chaplin, V.; Charles, E.; Chekhtman, A.; Chiang, J.; Ciprini, S.; Claus, R.; Cohen Tanugi, J.; Connaughton, V.; Conrad, J.; Cutini, S.; Dermer, C. D.; Angelis, A.; Palma, F.; Dingus, B. L.; E. D. C.,; Drell, P. S.; Dubois, R.; Favuzzi, C.; Fegan, S. J.; Ferrara, E. C.; Focke, W. B.; 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, I. A.; Grove, J. E.; Guiriec, S.; Hadasch, D.; Hanabata, Y.; Harding, A. K.; Hayashi, K.; Hayashida, M.; Hays, E.; Horan, D.; Hughes, R. E.; Itoh, R.; Johannesson, G.; Johnson, A. S.; Johnson, W. N.; Kamae, T.; Katagiri, H.; Kataoka, J.; Kippen, R. M.; Kn\odlseder, J.; Kocevski, D.; Kouveliotou, C.; Kuss, M.; Lande, J.; Latronico, L.; Lee, S.; Garde, M.; Longo, Francesco; Loparco, F.; Lovellette, M. N.; Lubrano, P.; Makeev, A.; Mazziotta, M. N.; Mcbreen, S.; Mcenery, J. E.; Mcglynn, S.; Meegan, C.; Mehault, J.; Meszaros, P.; Michelson, P. F.; Mizuno, T.; Monte, C.; Monzani, M. E.; Moretti, E.; Morselli, A.; Moskalenko, I. V.; Murgia, S.; Nakajima, H.; Nakamori, T.; Naumann Godo, M.; Nishino, S.; Nolan, P. L.; Norris, J. P.; Nuss, E.; Ohno, M.; Ohsugi, T.; Okumura, A.; Omodei, N.; Orlando, E.; Ormes, J. F.; Ozaki, M.; Paciesas, W. S.; Paneque, D.; Panetta, J. H.; Parent, D.; Pelassa, V.; Pepe, M.; Pesce Rollins, M.; Petrosian, V.; Piron, F.; Porter, T. A.; Preece, R.; Racusin, J. L.; Rain\`o, S.; Rando, R.; Rau, A.; Razzano, M.; Razzaque, S.; Reimer, A.; Reimer, O.; Reposeur, T.; Reyes, L. C.; Ripken, J.; Ritz, S.; Roth, M.; Ryde, F.; Sadrozinski, H. F.; Sander, A.; Scargle, J. D.; Schalk, T. L.; Sgr\`o, C.; Siskind, E. J.; Smith, P. D.; Spandre, G.; Spinelli, P.; Stamatikos, M.; Stecker, F. W.; Strickman, M. S.; Suson, D. J.; Tajima, H.; Takahashi, H.; Tanaka, T.; Tanaka, Y.; Thayer, J. B.; Thayer, J. G.; Tibaldo, L.; Tierney, D.; Toma, K.; Torres, D. F.; Tosti, G.; Tramacere, A.; Uchiyama, Y.; Uehara, T.; Usher, T. L.; Vandenbroucke, J.; Van, A. J.; Vasileiou, V.; Vilchez, N.; Vitale, V.; Kienlin, A.; Waite, A. P.; Wang, P.; Wilson Hodge, C.; Winer, B. L.; Wood, K. S.; X. F., 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.

Detection of a Spectral Break in the Extra Hard Component of GRB 090926A / Ackermann, M., Ajello, M., Asano, K., Axelsson, M., Baldini, L., Ballet, J., BARBIELLINI AMIDEI, G., Baring, M.G., Bastieri, D., Bechtol, K., Bellazzini, R., Berenji, B., Bhat, P.N., Bissaldi, E., Blandford, R.D., Bonamente, E., Borgland, A.W., Bouvier, A., Bregeon, J., Brez, A., et al.. - In: THE ASTROPHYSICAL JOURNAL. - ISSN 0004-637X. - 729:(2011), pp. 114-1-114-12. [10.1088/0004-637X/729/2/114]