GCN Circular 20946

A. Melandri (INAF-OAB), S. D. Barthelmy (GSFC), D. N. Burrows (PSU),
P. D'Avanzo (INAF-OAB), A. Y. Lien (GSFC/UMBC), D. M. Palmer (LANL)
and B. Sbarufatti (INAF-OAB/PSU) report on behalf of the Swift Team:

At 01:40:46 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170331A (trigger=744791).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 323.782, -24.414, which is 
   RA(J2000)  =  21h 35m 08s
   Dec(J2000) = -24d 24' 48"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a complicated set of overlapping
peaks with a total duration of about 25 sec.  The peak count rate
was ~1400 counts/sec (15-350 keV), at ~5 sec after the trigger. 

The XRT began observing the field at 01:42:50.6 UT, 124.2 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 323.79260, -24.38604 which
is equivalent to:
   RA(J2000)  = 21h 35m 10.22s
   Dec(J2000) = -24d 23' 09.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 105 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 in excess of the Galactic value (4.65 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.1
(+3.11/-2.65) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 127 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 0.05. 

Burst Advocate for this burst is A. Melandri (andrea.melandri AT brera.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/too.html.)