TITLE: GCN GRB OBSERVATION REPORT NUMBER: 4021 SUBJECT: HETE Fregate observations of GRB 050922C DATE: 05/09/23 01:18:28 GMT FROM: Roland Vanderspek at MIT HETE Fregate observations of GRB 050922C G. Crew, G. Ricker, J-L. Atteia, N. Kawai, D. Lamb, and S. Woosley, on behalf of the HETE Science Team; M. Arimoto, T. Donaghy, E. Fenimore, M. Galassi, C. Graziani, J. Kotoku, M. Maetou, M. Matsuoka, Y. Nakagawa, T. Sakamoto, R. Sato, Y. Shirasaki, M. Suzuki, T. Tamagawa, K. Tanaka, Y. Yamamoto, and A. Yoshida, on behalf of the HETE WXM Team; N. Butler, J. Doty, G. Prigozhin, R. Vanderspek, J. Villasenor, J. G. Jernigan, A. Levine, G. Azzibrouck, J. Braga, R. Manchanda, and G. Pizzichini, on behalf of the HETE Operations and HETE Optical-SXC Teams; M. Boer, J-F Olive, J-P Dezalay, and K. Hurley, on behalf of the HETE FREGATE Team; report: The HETE Fregate instrument detected GRB 050922C (Swift trigger 156647; Norris et al, GCN 4013). The flight instruments did not trigger because they were processing another trigger (H3934) which occurred 204 seconds earlier. The burst was clearly detected in Fregate in the 7-30, 7-80, and 30-400 keV bands. The burst spectrum is best fit with a cutoff power law of photon index 0.83 (-0.26/+0.23) and Epeak of 130.5 keV (-26.8/50.9). The 2-30 keV fluence is 5.4e-7 erg/cm2; the 30-400 keV fluence is 2.6e-6 erg/cm2. The ratio of the 2-30 keV to 30-400 keV fluence is 0.2: thus, GRB 050922C is a classical GRB. Due to the hardness of the spectrum, we have not been able to determine the high-energy slope (beta) of the Band function for this burst. The determination of bolometric fluence is therefore subject to systematic uncertainty. Assuming z=2.17 (Jakobbson et al., GCN 4017) and assuming the above cutoff power-law applies in the source-frame energy range of 1-10000 keV, the lower bound on E_iso is E_iso=3.7E+52 erg. The upper bound, obtained assuming beta=-2.0, is E_iso=7.0E+52 erg.