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GCN Circular 22589

Subject
GRB 180404A: Swift detection of a burst with an optical afterglow
Date
2018-04-04T00:54:52Z (6 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
J.D. Gropp (PSU), D. N. Burrows (PSU), A. Deich (PSU),
C. Gronwall (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 00:45:35 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180404A (trigger=821881).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 83.564, -37.170 which is 
   RA(J2000) = 05h 34m 15s
   Dec(J2000) = -37d 10' 10"
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 50 sec.  The peak count rate
was ~1900 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 00:47:02.1 UT, 86.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 83.54900, -37.16824 which is equivalent
to:
   RA(J2000)  = 05h 34m 11.76s
   Dec(J2000) = -37d 10' 05.7"
with an uncertainty of 4.2 arcseconds (radius, 90% containment). This
location is 43 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.  We
cannot determine whether the source is fading at the present time. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 89 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:34:11.63 =  83.54846
  DEC(J2000) = -37:10:03.9  = -37.16774
with a 90%-confidence error radius of about 0.65 arc sec. This position is 2.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.40 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is J.D. Gropp (jdg44 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/too.html.)
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