GCN Circular 20908

A. Cholden-Brown (PSU), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
J. A. Kennea (PSU), H. A. Krimm (CRESST/NSF/USRA),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 15:27:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170318B (trigger=743086).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 284.279, +6.269, which is 
   RA(J2000)  =  18h 57m 07s
   Dec(J2000) = +06d 16' 10"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a single peak
with a duration of about 3 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 15:29:06.3 UT, 73.7 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 284.30606, 6.29833
which is equivalent to:
   RA(J2000)  = 18h 57m 13.45s
   Dec(J2000) = +06d 17' 54.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 141 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 does not constrain the column density. 

The initial flux in the 2.5 s image was 5.39e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 84 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. 

Burst Advocate for this burst is A. Cholden-Brown (aaronb 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/too.html.)