T. Sakamoto (GSFC/ORAU), L. Barbier (GSFC), S. Barthelmy (GSFC),
J. Cummings (GSFC/UMBC), M. Stamatikos (GSFC/ORAU), T. Ukwatta (GWU),
K. McLean (GSFC/UMD), W. Baumgartner (GSFC/UMBC), E. Fenimore (LANL),
H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL),
G. Sato (GSFC/ISAS), J. Tueller (GSFC)
(i. e. the Swift-BAT team)
We report further results on GRB 080405 (Cummings et al. GCN circ
#7564) based on the BAT event data from T-1.3 to T+8.7 seconds.
This interval covers only about the first fifth of the event, and
not the interval of peak flux.
The best BAT position remains the same: RA, Dec 162.597, -4.250 deg
RA(J2000) = 10h 50m 23.3s,
Dec(J2000) = -4d 15' 00" (J2000)
with an uncertainty of 2.0 arcmin (radius, sys+stat, 90% containment).
The burst was 36% coded at T0. A Swift preplanned slew began at
about T+4 sec, and the burst left BAT's coded field of view entirely
at about T+45 sec. The unweighted counting rates are still somewhat
sensitive to photons entering through the side of the instrument.
These rates show a slow rise becoming detectable beginning about
T-4 seconds, rising to a series of overlapping peaks at T+12, T+20,
T+27, T+32, T+36, T+44, and T+47 seconds, with the peak at T+27
appearing the highest. The burst seems to follow an approximately
Gaussian envelope (though as noted, the profile is modulated by the
slew and by scattering and transmission through the side of the
instrument). T90 is 40 +-5 seconds. It fades below detectability at
about T+52 seconds.
The spectrum from T-1.3 to T+8.7 sec is well fit by a single
power-law function with a photon index of 1.28 +- 0.07. The fluence
is (1.2 +- 0.1) x 10^-6 ergs/cm2. We estimate that the total fluence
of the entire burst is ~5 x 10-5 ergs/cm2; i. e. we caught only a
small fraction of the burst in our detailed data.