TITLE:   GCN CIRCULAR
NUMBER:  34450
SUBJECT: GRB 230816A: Swift detection of a burst
DATE:    23/08/16 16:24:41 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>


M. H. Siegel (PSU), S. Dichiara (PSU),
R. A. J. Eyles-Ferris (U Leicester), J.D. Gropp (PSU),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), 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 16:08:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230816A (trigger=1185673).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 328.375, +37.828 which is 
   RA(J2000) = 21h 53m 30s
   Dec(J2000) = +37d 49' 43"
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 25 sec.  The peak count rate
was ~1900 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 16:10:18.4 UT, 90.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 328.44552,
37.85549 which is equivalent to:
   RA(J2000)  = 21h 53m 46.92s
   Dec(J2000) = +37d 51' 19.8"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 223 arcseconds from the BAT onboard position. This position
may be improved as more data are received; the latest position is
available at https://www.swift.ac.uk/sper. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (2.90 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.7
(+3.48/-2.91) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 93 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 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 0.311. 

Burst Advocate for this burst is M. H. Siegel (siegel 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/)