GCN Circular 35378

T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Dichiara (PSU),
M. Ferro (INAF-OAB), J.D. Gropp (PSU), S. Laha (GSFC/UMBC),
M. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 18:35:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 231216A (trigger=1202749).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 39.759, +33.596 which is 
   RA(J2000) = 02h 39m 02s
   Dec(J2000) = +33d 35' 45"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex
structure with a duration of about 70 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 18:37:53.1 UT, 131.1 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 39.77287, 33.57671 which
is equivalent to:
   RA(J2000)  = 02h 39m 05.49s
   Dec(J2000) = +33d 34' 36.2"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 80 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 8.36
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 250 seconds with the U filter starting
191 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 33% of the
XRT error circle. The typical 3-sigma upper limit has been about 19.2 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 0.081. 

Burst Advocate for this burst is T. Sbarrato (tullia.sbarrato AT inaf.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/)