GCN Circular 21042

A. P. Beardmore (U Leicester), N. P. M. Kuin (UCL-MSSL),
K. L. Page (U Leicester), D. M. Palmer (LANL),
B. Sbarufatti (INAF-OAB/PSU) and R. L. C. Starling (U Leicester)
report on behalf of the Swift Team:

At 09:13:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170428A (trigger=750298).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 330.063, +26.918 which is 
   RA(J2000) = 22h 00m 15s
   Dec(J2000) = +26d 55' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single peak
structure with a duration of about 0.25 sec.  The peak count rate
was ~22000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 09:15:06.9 UT, 84.6 seconds after
the BAT trigger. Swift then passed into the SAA, during which time no
data are collected.  No source was detected in 208 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of  84 seconds with the U filter starting
866 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 25% of the
BAT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The
8'x8' region for the list of sources generated on-board covers 100% of the BAT
error circle. The list of sources is typically complete to about 18.0 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.06. 

Burst Advocate for this burst is A. P. Beardmore (apb AT star.le.ac.uk). 
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.)