TITLE:   GCN CIRCULAR
NUMBER:  23182
SUBJECT: GRB 180828A: Swift detection of a GRB or possible Galactic Transient Swift J1754.9-2548
DATE:    18/08/28 19:27:00 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. P. Beardmore (U Leicester), S. W. K Emery (UCL-MSSL),
J. A. Kennea (PSU), N. P. M. Kuin (UCL-MSSL), S. J. LaPorte (PSU),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester),
D. M. Palmer (LANL) and A. Tohuvavohu (PSU) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 18:57:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located a source which is either GRB 180828A or a previously-unknown 
Galactic transient (trigger=856977).  
Swift slewed immediately to the source location. 
The BAT on-board calculated location is 
RA, Dec 268.700, -25.800 which is 
   RA(J2000) = 17h 54m 48s
   Dec(J2000) = -25d 47' 58"
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 15 sec.  The peak count rate
was ~20,000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 18:59:30.3 UT, 115.6 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 268.7184, -25.7991 which is equivalent to:
   RA(J2000)  = 17h 54m 52.42s
   Dec(J2000) = -25d 47' 56.8"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). No
event data are yet available to determine the column density using
X-ray spectroscopy. 

UVOT took a finding chart exposure of  30 seconds with the White filter
starting 123 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. The typical 3-sigma upper limit has been about 19.6 mag. 
Results from the list of sources generated on-board are not available at this
time. No correction has been made for the large, but uncertain, extinction
expected. 

Due to its location in the Galactic Bulge (3.8 degrees from Sgr A*,  
0.14 degrees below the plane), this trigger may be due to a previously
unknown X-ray transient. If this source is an X-ray transient, we 
name it Swift J1754.9-2548. Although the initial GCN Notice 
identified this as possibly related to Swift J1753.7-2544, the XRT 
position of this transient is 16.5 arcmin away from Swift 
J1753.7-2544, and therefore it is unrelated. 

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.)