GCN Circular 23149

F. E. Marshall (NASA/GSFC), V. D'Elia (ASDC),
S. W. K Emery (UCL-MSSL), J. A. Kennea (PSU),
M. J. Moss (George Washington University), K. L. Page (U Leicester)
and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 12:30:18 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180818B (trigger=853882).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 104.235, +39.329 which is 
   RA(J2000) = 06h 56m 56s
   Dec(J2000) = +39d 19' 43"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 130 sec, starting ~100 sec before
the trigger time during the previous spacecraft slew.  The peak count 
rate was ~1000 counts/sec (15-350 keV), at ~-90 sec after the trigger. 

The XRT began observing the field at 12:33:42.4 UT, 203.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 104.22468, 39.31662 which
is equivalent to:
   RA(J2000)  = 06h 56m 53.92s
   Dec(J2000) = +39d 18' 59.8"
with an uncertainty of 4.4 arcseconds (radius, 90% containment). This
location is 53 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.  We
cannot determine whether the source is fading at the present time. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

The initial flux in the 2.5 s image was 1.30e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 212 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.09. 

Burst Advocate for this burst is F. E. Marshall (marshall AT milkyway.gsfc.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.)