GCN Circular 40853

M. Ferro (INAF-OAB), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU),
R. Gupta (NASA GSFC) and D. M. Palmer (LANL) report on behalf of the
Neil Gehrels Swift Observatory Team:

At 00:55:56.71 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 250628A (trigger=1328546). Swift slewed immediately to the burst.
The BAT on-board calculated location is RA, Dec 38.551, -38.081 which is 
   RA(J2000) = 02h 34m 12s
   Dec(J2000) = -38d 04' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed two emission pulses with 
the first brighter pulse starting from T0-20 sec to T0+20 sec and 
the second fainter pulse from T0+25 end T0+60 sec.  The peak count rate
was ~1066 counts/sec (15-350 keV), at ~1 sec after the trigger.

The XRT began observing the field at 00:57:57.7 UT, 121.1 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 38.55240,
-38.07909 which is equivalent to:
   RA(J2000)  = 02h 34m 12.58s
   Dec(J2000) = -38d 04' 44.7"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 7.9 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.  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 in excess of the Galactic value (2.30 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.1
(+4.01/-3.17) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 9.47e-10 erg cm^-2 s^-1 (0.2-10
keV).

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 129 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
0.023.

Burst Advocate for this burst is M. Ferro (matteo.ferro 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/)