[vsnet-grb-info 2182] HETE Fregate observations of GRB 050922C

[GCN Circulars]

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  <roland at space.mit.edu>

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.