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GCN Circular 5823

GRB 061121: Swift detection of a bright burst with an optical counterpart
2006-11-21T15:39:45Z (18 years ago)
Scott Barthelmy at NASA/GSFC <>
K. L. Page (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), P. J. Brown (PSU),
D. N. Burrows (PSU), P. A. Evans (U Leicester),
N. Gehrels (NASA/GSFC), C. Guidorzi (Univ Bicocca&INAF-OAB),
J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA),
W. L. Landsman (NASA/GSFC), V. Mangano (INAF-IASFPA),
F. E. Marshall (NASA/GSFC), J. P. Osborne (U Leicester),
C. Pagani (PSU), D. M. Palmer (LANL), M. Perri (ASDC),
P. Romano (Univ. Bicocca & INAF-OAB), T. Sakamoto (NASA/ORAU),
M. Stamatikos (NASA/ORAU), R. L. C. Starling (U Leicester),
E. Troja (INAF-IASFPA), D. E. Vanden Berk (PSU) and
H. Ziaeepour (UCL-MSSL) report on behalf of the Swift Team:

At 15:22:29 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 061121 (trigger=239899).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 147.252, -13.183 {09h 49m 00s, -13d 10' 57"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows an initial pulse of
~3000 cnts/sec lasting ~10 sec.  Then a second much brighter peak starts
at ~T+50 sec, peaking at T+70 sec (at 32,000 cnts/sec), and ending at ~T+80 sec. 

The XRT began observing the field at 15:23:24 UT, 55 seconds after the
BAT trigger. XRT found a very bright, fading, uncatalogued X-ray
source, located at 
RA(J2000) = 09h 48m 54.5s, 
Dec(J2000) = -13d 11' 46.4"

with an estimated uncertainty of 4.7 arcseconds (90%
confidence radius). This is a ground calculated position based on
prompt downlinked data. This location is 102 arcseconds from the BAT
on-board position, within the BAT error circle. The initial flux in
the 2.5s image was 1.8e-08 erg/cm2/s (0.2-10 keV). There is a large
flare peaking at about 70 s after the BAT trigger. 

UVOT took a finding chart exposure of 526 seconds with the V filter starting
167 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	09:48:54.58 = 147.2274
  DEC(J2000) = -13:11:42.7  = -13.1952
with a 1-sigma error radius of about 0.5 arc sec. This position is 5.0 arc sec. 
from the center of the XRT error circle. The estimated magnitude is 17.2 with a
1-sigma error of about 0.5 mag. No correction has been made for the expected
extinction of about 0.1 magnitudes. 

We note that the BAT triggered on a precursor allowing the XRT 
and UVOT to observe during the main emission of the burst.
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