[vsnet-grb-info 26178] Swift Trigger 992444: a possible GRB 200821A

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28278
SUBJECT: Swift Trigger 992444: a possible GRB 200821A
DATE:    20/08/21 14:04:25 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

S. Dichiara (NASA/GSFC/UMCP), S. D. Barthelmy (GSFC),
D. N. Burrows (PSU), P. A. Evans (U Leicester), C. Gronwall (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), N. J. Klingler (PSU),
N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC/CRESST),
K. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 13:15:05 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a possible GRB 200821A (trigger=992444).  Swift slewed 
immediately to the location. 
The BAT on-board calculated location is 
RA, Dec 353.570, -48.277 which is 
   RA(J2000) = 23h 34m 17s
   Dec(J2000) = -48d 16' 37"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a sequence of
short peaks covering about 20 seconds.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~9 sec after the trigger. 

The XRT began observing the field at 13:17:33.3 UT, 147.9 seconds after
the BAT trigger. No source was detected in 1.3 ks of promptly
downlinked data. We are waiting for the full dataset to detect and
localise the XRT counterpart. 

UVOT took a finding chart exposure of 250 seconds with the U filter
starting  310 seconds after the BAT trigger. There is a candidate
afterglow in the  rapidly available 2.7'x2.7' sub-image at 
  RA(J2000)  =	23:34:24.51 = 353.60212 
  DEC(J2000) = -48:16:28.4  = -48.27456  with a 90%-confidence error
radius of about 0.83 arc sec. This position is 77.5  arc sec. from the
center of the BAT error circle. The estimated magnitude is  19.92 with
a 1-sigma error of about  0.23. No correction has been made for the 
expected extinction corresponding to E(B-V) of 0.01.  We note there is
a known faint source at this position in the DSS. 

This source lies within the current (Sector 28) field-of-view of TESS camera 2. 

This event has a highly unusual lightcurve for a GRB in BAT. 
The XRT data is dominated by a single hot pixel, which is preventing
rapid analysis of the data to find a counterpart.  The UVOT
detection may be due to the known DSS source. 

For these reasons we cannot confidently say whether this is an instrumental
artifact or an astrophysical source.  Further analysis will require the
downlinked data set. 

Burst Advocate for this burst is S. Dichiara (dichiara AT umd.edu). 
Please contact the BA by email if you require additional information
regarding Swift followup of this burst. In extremely urgent cases, after
trying the Burst Advocate, you can contact the Swift PI by phone (see
Swift TOO web site for information: http://www.swift.psu.edu/)