GCN Circular 3890

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 FREGATE instrument onboard HETE also detected the Swift burst GRB
050824 (Campana, et al., GCN 3866
Not found
 
 
) as an untriggered event (U11648)
with a significance of 6 sigma.  The direction to the burst source was
about 50 degrees from the instrument boresight, placing it outside the
WXM and SXC fields-of-view.

We have analyzed the spectrum of the event.  A power-law (PL) fit to
the full FREGATE energy range (6-400 keV) yields a best-fit power-law
index of 2.35 +0.88/-0.48 (90% CL), consistent with the PL index
reported by the Swift BAT team (Krimm et al., GCN 3871
Not found
 
 
).  Assuming the
best-fit PL index, the value of the fluence ratio S(2-30)/S(30-400) is
2.7.  GRB 050824 is thus an X-Ray Flash.

A PL fit restricted to photons with energy above 20 keV yields a
best-fit PL index of 3.29 +2.50/-1.36 (90% CL).  This steepening of 
the PL index suggests that the spectrum is representable by a Band
model with a value of E_peak near or below the low end of the FREGATE
energy range, so that the PL fit to the full energy range represents 
an average of the contributions from the high- and low-energy parts of 
the Band model.

Because of the low S/N of the event, a PL times exponential fit and a
Band-function fit are not statistically preferred over the simple PL
model.  Nonetheless, in order to constrain E_peak, we have performed a
"constrained-Band model" analysis of the spectrum (see Sakamoto et al.,
ApJ 602, 875), and find E_peak < 12.7 keV (90% CL).

FREGATE light curve data and plots will be available soon at the
following URL: http://space.mit.edu/HETE/Bursts/U11648.