GCN Circular 3183

H. Krimm (GSFC/USRA), L. Barbier, S. Barthelmy (GSFC),
J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (UMD), D.Q. Lamb (U. Chicago), C. Markwardt (GSFC/UMD),
D. Palmer (LANL), A. Parsons (GSFC), T. Sakamoto (GSFC/NRC),
G. Sato (ISAS), M. Suzuki (Saitama), M. Tashiro (Saitama U.),
J. Tueller (GSFC),  on behalf of the Swift/BAT team:

At 15:58:48.40 UT Swift-BAT detected GRB 050406 (trigger=113872)
(GCN Circ 3180, Parsons et al.). The refined BAT ground position is
(RA,Dec) = 34.471 -50.181, [deg; J2000] +- 3 arcmin, (95%
containment).  This is 28 arc seconds from the position determined
on-board and originally reported.

Examination of the mask weighted light curve confirms that only the initial
peak is associated with the burst.  The other possible emission reported
in GCN Circ 3180 is attributed to background fluctuations.
The shape of the peak is fast-rise, exponential decay in the 15-25 keV
band.  In the 25-50 keV band, the peak starts ~2 secs earlier and the
shape is more symmetric. We derive T90 (15-25 keV) =  3 seconds
+/- 1 s, and T90 (15-350 keV) = 5 seconds +/- 1 s. Errors on T90
include systematics.

Analysis of the event data shows that this is a very soft burst with
no significant flux above 50 keV.   Plotting GRB 050406 on a color-color
diagram indicates that this burst may have the characteristics of an
X-ray Flash.  The fluence derived from the event data is
9.0 X 10^-8 erg/cm^2 in the 15-350 keV band, and
4.8 X 10^-8 erg/cm^2 in the 15-50 keV band.  The 1-s peak flux
(T+0.6 s) is 3.2 ph/cm^2/s  (15-25 keV).  The photon index of the
1-s peak spectrum (T+0 s) is 2.32 +/- 0.53 (90% confidence).
The time-averaged spectrum yields a photon index of 2.38 +/- 0.34
(90% confidence). Both the 1-s and time-averaged spectra are well
fit by a simple power-law.