GCN Circular 34479

A. Tohuvavohu (U Toronto), S. Dichiara (PSU), J.D. Gropp (PSU),
D. M. Palmer (LANL), T. M. Parsotan (GSFC/UMBC/CRESSTII),
B. Sbarufatti (INAF-OAB) and M. A. Williams (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 23:27:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230818A (trigger=1186032).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 285.883, +40.876 which is 
   RA(J2000) = 19h 03m 32s
   Dec(J2000) = +40d 52' 35"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single complex
peak structure with a duration of about 10 sec.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 23:29:55.4 UT, 141.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 285.88966, 40.89504 which is equivalent
to:
   RA(J2000)  = 19h 03m 33.52s
   Dec(J2000) = +40d 53' 42.1"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 70 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 https://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 1.13
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 144 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. 
Results from the list of sources generated on-board are not available at this
time. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.101. 

Burst Advocate for this burst is A. Tohuvavohu (aaron.tohu 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/)