GCN Circular 3661

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

At 04:29:14.28 UT Swift-BAT detected GRB 050721 (trigger=146970)
(GCN Circ 3654, A. Antonelli et al.).  This burst has been called
GRB 050721A in some circulars, although it is so far the only bona fide
burst of the day.  The refined BAT ground position is (RA,Dec) =
253.452, -28.386 {16h 53m 48.5s, -28d 23' 10"} (J2000) +- 3 arcmin
(radius, 90% containment, stat+sys).  The partial coding was 13%.

The masked-weighted light curve shows a FRED-like structure with a
single large peak starting to rise at T-5 seconds, peaking at T+3.7 seconds
and decaying back to background levels by T+50 seconds.  The peak
is visible in the 15-100 keV energy band, but not at higher energies.
The calculated T90 (15-350 keV) is 39 +- 2 sec (estimated error
including systematics).

The power-law photon index of the time-averaged spectrum is 1.81 +- 0.08.
The fluence in the 15-350 keV band is (5.05 +- 0.25) x 10^-6 erg/cm2.
The 1-sec peak photon flux measured from T+3.7 sec in the 15-350
band is (3.4 +- 0.8) ph/cm2/sec.  All the quoted errors are at the 90%
confidence level.