TITLE:   GCN CIRCULAR
NUMBER:  22798
SUBJECT: GRB 180620A: Swift detection of a burst with an optical counterpart
DATE:    18/06/20 08:50:56 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

P. A. Evans (U Leicester), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), A. A. Breeveld (UCL-MSSL),
S. W. K Emery (UCL-MSSL), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL),
R. L. C. Starling (U Leicester) and A. Tohuvavohu (PSU) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 08:34:58 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180620A (trigger=843122).  Swift slewed to the burst with a 3 min delay. 
The BAT on-board calculated location is 
RA, Dec 279.883, +23.265, which is 
   RA(J2000)  =  18h 39m 32s
   Dec(J2000) = +23d 15' 55"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows several overlapping peaks
with a total duration of about 25 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~12 sec after the trigger. 

The XRT began observing the field at 08:39:25.1 UT, 267.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 279.89575, 23.24373 which is equivalent
to:
   RA(J2000)  = 18h 39m 34.98s
   Dec(J2000) = +23d 14' 37.4"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 87 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 1.60
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 270 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
  RA(J2000)  =	18:39:35.08 = 279.89618
  DEC(J2000) = +23:14:39.7  =  23.24437
with a 90%-confidence error radius of about 1.10 arc sec. This position is 1.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.24. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.13. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 
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.)