GCN Circular 18087

H. A. Krimm (CRESST/GSFC/USRA), S. D. Barthelmy (GSFC),
A. A. Breeveld (UCL-MSSL), D. N. Burrows (PSU), S. B. Cenko (GSFC),
P. A. Evans (U Leicester), C. Gronwall (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
P. Romano (INAF-IASFPA), T. Sakamoto (AGU),
B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf
of the Swift Team:

At 12:51:11 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 150728A (trigger=650617).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 292.288, +33.904 which is 
   RA(J2000)  =  19h 29m 09s
   Dec(J2000) = +33d 54' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows two weak peaks
with a total duration of about 20 sec.  The peak count rate
was ~1000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 12:52:21.1 UT, 70.2 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 292.2269, 33.9165 which
is equivalent to:
   RA(J2000)  = 19h 28m 54.46s
   Dec(J2000) = +33d 54' 59.4"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 188 arcseconds from the BAT onboard position, outside 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 2.59
x 10^21 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 75 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  0.20. 

Burst Advocate for this burst is H. A. Krimm (hans.krimm 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.)