GCN Circular 22573

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), A. Deich (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), K. L. Page (U Leicester) and
A. Tohuvavohu (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 15:38:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180331B (trigger=820513).  Swift executed a delayed slewed
due to an observing constraint.  The BAT on-board calculated location is 
RA, Dec 196.820, +61.725, which is 
   RA(J2000)  =  13h 07m 17s
   Dec(J2000) = +61d 43' 30"
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 8 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 15:49:43.0 UT, 663.7 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 196.7829, 61.7271 which
is equivalent to:
   RA(J2000)  = 13h 07m 07.91s
   Dec(J2000) = +61d 43' 37.5"
with an uncertainty of 2.4 arcseconds (radius, 90% containment). This
location is 63 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.  We
cannot determine whether the source is fading at the present time. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 667 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 expected extinction corresponding to E(B-V) of
0.01. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien 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.)