TITLE:   GCN CIRCULAR
NUMBER:  22790
SUBJECT: GRB 180618A: Swift detection of a burst with an optical counterpart
DATE:    18/06/18 00:53:53 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

S. J. LaPorte (PSU), S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
D. M. Palmer (LANL), M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report
on behalf of the Neil Gehrels Swift Observatory Team:

At 00:43:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180618A (trigger=842475).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 169.937, +73.848 which is 
   RA(J2000) = 11h 19m 45s
   Dec(J2000) = +73d 50' 52"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single FRED
structure with a duration of about 20 sec.  The peak count rate
was ~10000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 00:44:30.5 UT, 77.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 169.9415, 73.8358 which is equivalent to:
   RA(J2000)  = +11h 19m 45.96s
   Dec(J2000) = +73d 50' 08.9"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). This
location is 44 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. 

The initial flux in the 2.5 s image was 1.27e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 86 seconds after the BAT trigger. There is a candidate afterglow in
the list of sources generated on-board at
  RA(J2000)  =	11:19:45.78 = 169.94074
  DEC(J2000) = +73:50:13.3  =  73.83704
with a 90%-confidence error radius of about 1.10 arc sec. This position is 4.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.81. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.07. 

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