TITLE:   GCN CIRCULAR
NUMBER:  22148
SUBJECT: GRB 171123A: Swift detection of a burst
DATE:    17/11/23 14:54:37 GMT
FROM:    Phil Evans at U of Leicester  <pae9@star.le.ac.uk>

P. A. Evans (U Leicester), D. N. Burrows (PSU), A. Deich (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
M. H. Siegel (PSU) and A. Tohuvavohu (PSU) report on behalf of the
Swift Team:

At 14:14:47 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 171123A (trigger=791922).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 186.190, +32.531 which is 
   RA(J2000) = 12h 24m 46s
   Dec(J2000) = +32d 31' 51"
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 40 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 14:16:17.8 UT, 90.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
possible X-ray source with an enhanced position: RA, Dec 186.2425, 32.5032
which is equivalent to:
   RA(J2000)  = 12h 24m 58.19s
   Dec(J2000) = +32d 30' 11.4"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 188 arcseconds from the BAT onboard position, just outside the
BAT error circle. This possible source is weak and at the edge of the
region of the XRT which is promptly downlinked; we are not completely
certain that this is an astrophysical object. We are awaiting the full
dataset to confirm the nature of this source. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.90 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 9.4
(+9.50/-2.54) 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 0.02. 

Burst Advocate for this burst is P. A. Evans (pae9 AT star.le.ac.uk). 
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.)