GCN Circular 21960

S. B. Cenko (GSFC), S. D. Barthelmy (GSFC), P. D'Avanzo (INAF-OAB),
J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU)
and A. Tohuvavohu (PSU) report on behalf of the Swift Team:

At 02:37:00 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 171003A (trigger=775946).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 40.910, +61.408, which is 
   RA(J2000)  =  02h 43m 38s
   Dec(J2000) = +61d 24' 28"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  As is typical for image triggers, the real-time
light curve does not show anything significant. 

The XRT began observing the field at 02:43:29.0 UT, 388.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 40.9186, 61.4323 which is equivalent to:
   RA(J2000)  = +02h 43m 40.46s
   Dec(J2000) = +61d 25' 56.3"
with an uncertainty of 5.4 arcseconds (radius, 90% containment). This
location is 88 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.29e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 400 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 large, but uncertain extinction
expected. The analysis is complicated by the proximity of a known star
with a USNO V magnitude of 12.01 approximately 7.7 arc-seconds from
the XRT position. 

Based on this source's proximity to the Galactic plane 
(latitude=1.4 degrees) and the duration of the BAT image trigger
(>5 minutes), there is a possibility that this is a
Galactic transient (i.e. Swift J0243.6+6124) rather than a GRB.  
The nature of this source will be determined by the full downloaded
dataset and further observations. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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.)