TITLE:   GCN CIRCULAR
NUMBER:  34282
SUBJECT: GRB 230728A: Swift detection of a burst
DATE:    23/07/28 03:07:03 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>


C. Salvaggio (INAF-OAB), P. D'Avanzo (INAF-OAB), S. Laha (GSFC/UMBC),
D. M. Palmer (LANL), T. M. Parsotan (GSFC/UMBC/CRESSTII) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 02:50:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230728A (trigger=1181187).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 343.539, +28.151 which is 
   RA(J2000) = 22h 54m 09s
   Dec(J2000) = +28d 09' 03"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, BAT lightcurve information
is not immediately available. 

The XRT began observing the field at 02:51:33.5 UT, 90.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 343.51530,
28.17470 which is equivalent to:
   RA(J2000)  = 22h 54m 03.67s
   Dec(J2000) = +28d 10' 28.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 113 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
https://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 (6.29 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 7.2
(+2.52/-2.21) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 7.13e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 99 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. 
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
0.079. 

Burst Advocate for this burst is C. Salvaggio (chiara.salvaggio AT inaf.it). 
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/)