[vsnet-grb-info 3671] GRB 060904B: Swift detection of a burst with optical counterpart

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5505
SUBJECT: GRB 060904B: Swift detection of a burst with optical counterpart
DATE:    06/09/04 02:51:23 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott at lheamail.gsfc.nasa.gov>

D. Grupe (PSU), S. D. Barthelmy (GSFC), M. M. Chester (PSU),
J. R. Cummings (NASA/UMD), S. T. Holland (GSFC/USRA),
S. D. Hunsberger (PSU), J. A. Kennea (PSU), H. A. Krimm (GSFC/USRA),
C. B. Markwardt (GSFC/UMD), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), M. Stamatikos (NASA/ORAU) and M. C. Stroh (PSU)
report on behalf of the Swift Team:

At 02:31:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 060904B (trigger=228006).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA,Dec 58.213, -0.720 {03h 52m 51s, -00d 43' 11"} (J2000)
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows an initial 5-sec wide
peak at T+1 sec followed by an ~30-sec wide weaker peak at T+150 sec. 
The total duration is about 180 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 02:32:12 UT, 69 seconds after the
BAT trigger. XRT found a variable, uncatalogued X-ray source located
at RA(J2000) = 03h 52m 50.0s, Dec(J2000) = -00d 43' 32.8", with an
estimated uncertainty of 6.5 arcseconds (90% confidence radius). This
location is 26 arcseconds from the BAT on-board position, within the
BAT error circle. The initial flux in the 2.5s image was 4.3e-10
erg/cm2/s (0.2-10 keV). The source shows a rapid increase in count
rate indicating a bright flare that is possibly correlated with the 
second bright peak in the BAT. 

UVOT took a finding chart exposure of 246 seconds with the V filter
starting 70 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at (RA,DEC) (J2000) of
(58.2106,-0.7253) or (03h52m50.54s,-00o43'31.1")  with a 1-sigma
error radius of about 0.7 arc sec. This position is 7.7 arc sec. from
the center of the XRT error circle. The estimated magnitude is 18.6
with a 1-sigma error of about 0.5 mag. No correction has been made
for the expected extinction of about 0.6 magnitudes.