GCN Circular 3053

HETE-2 Observations of GRB 050215b (=U11570): A Likely X-Ray Flash

Y. Nakagawa, G. Ricker, J-L. Atteia, N. Kawai, D. Lamb, and S. Woosley,
on behalf of the HETE Science Team;

T. Donaghy, E. Fenimore, M. Galassi, C. Graziani, M. Matsuoka,
T. Sakamoto, R. Sato, Y. Shirasaki, M. Suzuki, T. Tamagawa, Y. Urata,
T. Yamazaki, Y. Yamamoto, and A. Yoshida, on behalf of the HETE WXM
Team;

N. Butler, G. Crew, 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:

GRB 050215b (=U11570) occurred 36 degrees off the HETE boresight, and
therefore well within the FREGATE field of view but only at the very
edge of the WXM field of view.  Consequently, the burst is seen in the
FREGATE data but not in the WXM data.

Using a simple power-law model to fit the FREGATE spectral data, we
find a best-fit power-law index of beta = -2.2 (+0.4/-0.6).  Using a
constrained Band model (Sakamoto et al., ApJ, 602, 875, 2004) to fit
the data, we find a best-fit value for the peak energy of E_peak =
17.6 keV; a 68% confidence region 4.8 keV < E_peak < 23.7 keV, a 95%
upper bound E_peak < 30.3 keV, and a 99% upper bound E_peak < 35.7 keV.
Finally, using a Band model with the low-energy power-law index alpha
fixed at -1 to fit the data, we find fluences of S_x (2-30 keV) = 2.8
(+3.3/-1.1) x 10-7 erg cm-2 and S_gamma (30-400 keV) = 1.7 (+2.5/-1.3)
x 10-7 erg cm-2.  The best-fit value of S_x/S_gamma is therefore 1.65.
Taken together, these results are strong evidence that GRB 050215b was
an X-Ray Flash.

We note that a supernova component has not been definitively
established in the afterglow of any X-Ray Flash and that any supernova
component in the afterglow of GRB 050215b would be expected to peak
soon.  We therefore encourage further observations of the afterglow of
this burst.

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