GCN Circular 23493

E. Troja (NASA/GSFC/UMCP), A. P. Beardmore (U Leicester),
A. A. Breeveld (UCL-MSSL), J.D. Gropp (PSU), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
M. J. Moss (George Washington University), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 12:08:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 181203A (trigger=874475).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 108.369, -39.769 which is 
   RA(J2000) = 07h 13m 29s
   Dec(J2000) = -39d 46' 07"
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 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:10:01.1 UT, 73.3 seconds after
the BAT trigger. The position determined from promptly downlinked data
differs significantly from the on-board position, suggesting that the
XRT may have centroided on a cosmic ray; the initial XRT position
notice should be treated with caution. Using promptly downlinked data
we find a fading, uncatalogued X-ray source with an enhanced position:
RA, Dec 108.3491, -39.8016 which is equivalent to:
   RA(J2000)  = 07h 13m 23.78s
   Dec(J2000) = -39d 48' 05.6"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 129 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.26
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 79 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
0.24. 

Burst Advocate for this burst is E. Troja (eleonora.troja AT 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.)