GCN Circular 4126

A. Retter (PSU), S. Barthelmy (GSFC), D. Burrows (PSU),
M. Chester (PSU), N. Gehrels (GSFC), J. Kennea (PSU),
F. Marshall (GSFC), D. Palmer (LANL), J. Racusin (PSU)
on behalf of the Swift team:

At 23:31:53 UT, Swift-BAT triggered and located GRB 051021B (trigger=160672).
The BAT on-board calculated location is RA,Dec 126.037d,-45.519d
{8h 24m 09s,-45d 31' 09"} (J2000), with an uncertainty of 3 arcmin
(radius, 90% containment, stat+sys).  The BAT light curve shows
a single FRED-like peak with structure with a total duration of ~25 sec.
There is some pre-trigger emission starting at T-8 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~2 seconds after the trigger.

The spacecraft slewed immediately and the XRT began observing at 23:33:14 UT
(81 sec after the BAT trigger).  The on-board centroiding algorithm was not
able to determine a position, but the prompt lightcurve and spectrum strongly
suggest that a fading source is present in the field of view.  Further analysis
will require ground-processed data.

UVOT began observing 23:33:11 UT or 78 sec after the BAT trigger
with a 200 sec V-band image.  It is a crowded field with high,
but uncertain extinction.  No new sources are seen in either
the small image or the list of sources produced in the on-board processing.
The limiting magnitude in the small image is about 19th mag.
The on-board list of sources covers the entire BAT error region and
is typically complete to about 17th mag.

Further analysis will be delayed due the lack of telemetry downlink passes
for several hours.