GCN Circular 18152

V. D'Elia (ASDC), A. A. Breeveld (UCL-MSSL),
P. A. Evans (U Leicester), N. Gehrels (NASA/GSFC),
L. Izzo (URoma/ICRA), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester) and B. Sbarufatti (INAF-OAB/PSU) report on
behalf of the Swift Team:

At 11:36:32 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150818A (trigger=652603).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 230.446, +68.342 which is 
   RA(J2000) = 15h 21m 47s
   Dec(J2000) = +68d 20' 32"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 50 sec.  The peak count rate
was ~1990 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 11:37:57.3 UT, 84.5 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 230.3621, 68.3446 which is equivalent to:
   RA(J2000)  = +15h 21m 26.90s
   Dec(J2000) = +68d 20' 40.6"
with an uncertainty of 6.1 arcseconds (radius, 90% containment). This
location is 111 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. 

UVOT took a finding chart exposure of  28 seconds with the White
filter  starting 94 seconds after the BAT trigger. No credible
afterglow candidate has  been found in the initial data products. The
2.7'x2.7' sub-image processing  failed because of no aspect solution
is available.   Results from the list of sources  generated on-board
are not available at this time. 

Burst Advocate for this burst is V. D'Elia (delia AT asdc.asi.it). 
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.)