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GCN Circular 22590

GRB 180404B: Swift detection of a burst with an optical counterpart
2018-04-04T02:23:56Z (6 years ago)
David Palmer at LANL <>
S. J. LaPorte (PSU), C. Gronwall (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 02:11:44 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180404B (trigger=821902).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 53.384, -50.195 which is 
   RA(J2000) = 03h 33m 32s
   Dec(J2000) = -50d 11' 41"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~9 sec after the trigger. 

The XRT began observing the field at 02:12:53.9 UT, 69.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 53.3967, -50.2151 which is equivalent to:
   RA(J2000)  = +03h 33m 35.21s
   Dec(J2000) = -50d 12' 54.4"
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 78 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 81 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	03:33:34.24 =  53.39265
  DEC(J2000) = -50:12:54.1  = -50.21504
with a 90%-confidence error radius of about 0.76 arc sec. This position is 9.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.61 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is S. J. LaPorte (extragsam AT 
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:
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