GCN Circular 17766

K. L. Page (U Leicester), M. M. Chester (PSU),
F. E. Marshall (NASA/GSFC), T. Sakamoto (AGU) and M. H. Siegel (PSU)
report on behalf of the Swift Team:

At 03:12:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150428B (trigger=639278).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 292.654, +4.116 which is 
   RA(J2000) = 19h 30m 37s
   Dec(J2000) = +04d 06' 57"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a double-peaked 
structure with a duration of about 30 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 03:13:50.9 UT, 107.5 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 292.63764, 4.12529
which is equivalent to:
   RA(J2000)  = 19h 30m 33.03s
   Dec(J2000) = +04d 07' 31.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 67 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 consistent with the Galactic value of 3.26
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 8.91e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 117 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. Because of the density of catalogued stars, further
analysis is required to report an upper limit for any afterglow in the
sub-image. The 8'x8' region for the list of sources generated on-board covers
100% of the XRT error circle. Because of the density of catalogued stars,
further analysis is required to report an upper limit for any afterglow in the
region. No correction has been made for the expected extinction corresponding
to E(B-V) of 0.40. 

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.)