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GCN Circular 23105

Subject
GRB 180809B: Swift detection of a burst with an optical afterglow
Date
2018-08-09T20:40:30Z (6 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
M. J. Moss (George Washington University), S. D. Barthelmy (GSFC),
V. D'Elia (ASDC), C. Gronwall (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 20:28:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180809B (trigger=852553).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 299.712, -15.302, which is 
   RA(J2000)  =  19h 58m 51s
   Dec(J2000) = -15d 18' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows multiple bright peaks
structure with a total duration of about 80 sec.  The peak count rate
was ~35,000 counts/sec (15-350 keV), at ~35 sec after the trigger. 

XRT found a bright, uncatalogued X-ray source located at RA, Dec
299.6998, -15.2988 which is equivalent to:
   RA(J2000)  = +19h 58m 47.95s
   Dec(J2000) = -15d 17' 55.7"
with an uncertainty of 6.1 arcseconds (radius, 90% containment). No
event data are yet available to determine the column density using
X-ray spectroscopy. 

The initial flux in the 0.1 s image was 1.51e-08 erg cm^-2 s^-1 (0.2-10
keV). 

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)  =	19:58:47.96 = 299.69983
  DEC(J2000) = -15:17:56.9  = -15.29915
with a 90%-confidence error radius of about 0.64 arc sec. This position is 1.3
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.91 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.16. 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 
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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