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GCN Circular 29192

Subject
GRB 201229A: Swift detection of a burst with a possible optical counterpart
Date
2020-12-29T11:23:17Z (3 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
B. Sbarufatti (PSU), S. D. Barthelmy (GSFC), J.D. Gropp (PSU) and
A. Y. Lien (GSFC/UMBC) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 10:59:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 201229A (trigger=1015088).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 210.708, +48.197 which is 
   RA(J2000) = 14h 02m 50s
   Dec(J2000) = +48d 11' 50"
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 7 sec.  The peak count rate
was ~500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 11:01:22.2 UT, 111.4 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 210.6891,
48.1976 which is equivalent to:
   RA(J2000)  = 14h 02m 45.39s
   Dec(J2000) = +48d 11' 51.2"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 45 arcseconds from the BAT onboard position, within the BAT
error circle. This position may be improved as more data are received;
the latest position is available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.78
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 116 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	14:02:45.41 = 210.68921
  DEC(J2000) = +48:11:53.8  =  48.19828
with a 90%-confidence error radius of about 0.62 arc sec. This position is 3.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.19 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is B. Sbarufatti (bxs60 AT psu.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/)
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