GCN Circular 17765

K. L. Page (U Leicester), M. M. Chester (PSU), N. Gehrels (NASA/GSFC),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL),
F. E. Marshall (NASA/GSFC), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 01:30:40 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150428A (trigger=639275).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 188.543, +6.981 which is 
   RA(J2000) = 12h 34m 10s
   Dec(J2000) = +06d 58' 51"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex structure 
with a duration of about 50 sec.  The peak count rate
was ~1700 counts/sec (15-350 keV), at ~17 sec after the trigger. 

The XRT began observing the field at 01:33:36.7 UT, 175.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 188.53889, 6.95211 which is equivalent
to:
   RA(J2000)  = 12h 34m 09.33s
   Dec(J2000) = +06d 57' 07.6"
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 (1.63 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 9.3
(+6.64/-5.03) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 179 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 0.02. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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.)