GCN Circular 16709

A. Vargas (PSU), D. N. Burrows (PSU), S. B. Cenko (GSFC),
N. Gehrels (NASA/GSFC), C. Gronwall (PSU), L. M. Z. Hagen (PSU),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC), J. L. Racusin (NASA/GSFC),
B. Sbarufatti (INAF-OAB/PSU) and C. A. Swenson (PSU) report on behalf
of the Swift Team:

At 18:44:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 140818B (trigger=609885).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 271.148, -1.361 which is 
   RA(J2000) = 18h 04m 35s
   Dec(J2000) = -01d 21' 40"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 18:46:05.6 UT, 109.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 271.13656, -1.38660 which is equivalent
to:
   RA(J2000)  = 18h 04m 32.77s
   Dec(J2000) = -01d 23' 11.8"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 100 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 3.53
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 113 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 none of
the XRT error circle. The 8'x8' region for the list of sources generated
on-board covers 100% of the BAT error circle. Because of the density of
catalogued stars, further analysis is required to report an upper limit for any
afterglow in the region. No correction has been made for the expected
extinction corresponding to E(B-V) of 0.71. 

Burst Advocate for this burst is A. Vargas (avargas AT swift.psu.edu). 
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.)