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GCN Circular 23852

Subject
GRB 190204A: Swift detection of a burst with a possible optical counterpart
Date
2019-02-04T06:12:47Z (5 years ago)
From
Aaron Tohuvavohu at PSU/Swift <aaronb@swift.psu.edu>
A. Y. Lien (GSFC/UMBC), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), N. J. Klingler (PSU), A. Melandri (INAF-OAB),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 05:46:01 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 190204A (trigger=887579 and 887580). 
Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 351.514, +54.878 which is 
   RA(J2000) = 23h 26m 03s
   Dec(J2000) = +54d 52' 42"
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 20 sec.  The peak count rate
was ~28000 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 05:47:15.7 UT, 75.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 351.47755, 54.87623 which
is equivalent to:
   RA(J2000)  = 23h 25m 54.61s
   Dec(J2000) = +54d 52' 34.4"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 119 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
http://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 4.36
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 5.72e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
295 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	23:25:54.58 = 351.47742
  DEC(J2000) = +54:52:34.9  =  54.87637
with a 90%-confidence error radius of about 0.63 arc sec. This position is 4.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.76 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.38. 

This GRB resulted in two BAT triggers in the on-board processing (#887579 at 05:46:01 and #887580 at 05:46:06).  
We adopt the first trigger number and time as T0. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT nasa.gov). 
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: /too.html.)
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