GCN Circular 3479

J. Cummings (GSFC/NRC), L. Barbier (GSFC), S. Barthelmy (GSFC),
M. Chester (PSU), E. Fenimore (LANL), N. Gehrels (GSFC),
D. Hullinger (UMD), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD),
F. Marshall (GSFC), T. Mitani (ISAS), D. Palmer (LANL),
A. Parsons (GSFC), T. Sakamoto (GSFC/NRC), G. Sato (ISAS),
M. Suzuki (Saitama), J. Tueller (GSFC),

on behalf of the Swift - BAT team:

At 00:02:53 UT Swift-BAT detected GRB 050525A (trigger=130088) (GCN
Circ 3466, Band, et al.). The refined BAT ground position is (RA,Dec)
= 278.140,+26.344, [deg; J2000] {18:32:34, +26:20:38} +- 0.5 arcmin,
(95% containment).  This position is consistent with the reported
optical transient positions to within 20 arcsec (GCN Circ 3468, 
Rykoff et al.; GCN Circ 3470, Torii et al.).

The image significance is 148 sigma, making GRB 050525 the most
significant BAT-imaged GRB to date.  The burst was 26 degrees off
axis, and the partial coding fraction was 85%.

The mask-weighted light curve shows two peaks with duration of 2
and 5 seconds respectively.  Each peak has two sub-pulses, which
become more distinct at higher energies.  T90 is (8.8 +- 0.5) seconds
and T50 is (5.2 +- 0.5) seconds (15-350 keV; estimated errors include
systematics).

The spectrum has significant curvature, and is not consistent with a
single power law model.  Fitting to a cut-off power law yields a
low-energy photon index of 1.0 +- 0.1, Epeak is 79 +- 4 keV.  Fitting 
to a "Band" GRB spectral model yields the same spectral parameters, 
but the "beta" index is unconstrained.  Chi2 for the Band model is
the same as for the cutoff power law.  The fluence in the 15-350 keV 
band is (2.0 +- 0.1) x 10^-5 erg/cm2.  The 1-s peak photon flux, 
starting at T0+1.63 second in the 15-350 keV band is (48 +- 1) 
ph/cm2/s.  All the quoted errors are at the 90% confidence level.