TITLE:   GCN CIRCULAR
NUMBER:  21371
SUBJECT: GRB 170728B: Swift detection of a short burst
DATE:    17/07/28 23:24:58 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

S. B. Cenko (GSFC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), A. Cholden-Brown (PSU),
J. A. Kennea (PSU), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester) and M. H. Siegel (PSU) report on behalf of
the Swift Team:

At 23:03:19 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 170728B (trigger=765130).  Due to an observing constraint,
Swift executed a 6-min delayed slew to the burst.  The BAT on-board calculated
location is RA, Dec 238.044, +70.142, which is 
   RA(J2000)  =  15h 52m 11s
   Dec(J2000) = +70d 08' 32"
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 1 sec.  The peak count rate
was ~5500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 23:11:08.2 UT, 468.9 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 237.9813, 70.1222 which
is equivalent to:
   RA(J2000)  = 15h 51m 55.52s
   Dec(J2000) = +70d 07' 19.9"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 104 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. 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 473 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 0.03. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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.)