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GCN Circular 26534

Subject
GRB 191221B: Swift detection of a burst and a very bright optical candidate
Date
2019-12-21T21:00:24Z (4 years ago)
From
Sibasish Laha at GSFC <sib.laha@gmail.com>
S Laha (GSFC/UMBC/CRESST), S. D. Barthelmy (GSFC), J.D. Gropp (PSU),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
K. K. Simpson (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 20:39:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191221B (trigger=945521).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 154.832, -38.141 which is 
   RA(J2000) = 10h 19m 20s
   Dec(J2000) = -38d 08' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 15 sec.  The peak count rate
was ~17000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 20:40:39.3 UT, 86.2 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 154.8286, -38.1579 which is equivalent to:
   RA(J2000)  = 10h 19m 18.86s
   Dec(J2000) = -38d 09' 28.4"
with an uncertainty of 6.8 arcseconds (radius, 90% containment). This
location is 61 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 91 seconds after the BAT trigger. A very bright afterglow candidate has
been found at the XRT position in the initial data products. Because of 
coincidence loss we cannot calculate the magnitude accurately at this time. 
No correction has been made for the expected extinction corresponding to E(B-V) of 0.08. 

Burst Advocate for this burst is S Laha (sib.laha AT gmail.com). 
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/)
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