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GCN Circular 22558

Subject
GRB 180329B: Swift detection of a burst with an optical afterglow
Date
2018-03-29T14:21:48Z (6 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
J. L. Racusin (NASA/GSFC), A. Deich (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 14:08:23 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180329B (trigger=819490).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 82.916, -23.708 which is 
   RA(J2000) = 05h 31m 40s
   Dec(J2000) = -23d 42' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a small complex
peak from T-10 to T+20 s (peak of 800 counts/s) with a stronger peak
from T+140 to T+160 (the limit of currently available data).  
The peak count rate was ~1400 counts/sec (15-350 keV), at ~147 sec 
after the trigger. 

The XRT began observing the field at 14:10:07.5 UT, 103.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 82.9031, -23.6889 which is equivalent to:
   RA(J2000)  = +05h 31m 36.74s
   Dec(J2000) = -23d 41' 20.0"
with an uncertainty of 6.5 arcseconds (radius, 90% containment). This
location is 80 arcseconds from the BAT onboard position, within the BAT
error circle. 

The initial flux in the 2.5 s image was 4.57e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 111 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:31:36.90 =  82.90376
  DEC(J2000) = -23:41:25.9  = -23.69052
with a 90%-confidence error radius of about 0.62 arc sec. This position is 6.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.65 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

The brighter, second, peak of this GRB occurs during the time that
XRT and UVOT were taking data, so the early observations sample
the prompt burst emission. 

Burst Advocate for this burst is J. L. Racusin (judith.racusin 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: http://www.swift.psu.edu/too.html.)
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