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; Silva EDCE; 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; Sadrozinski HFW; 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; Wu XF; 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.