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GCN Circular 30508

Subject
GRB 210725B: Swift detection of a burst
Date
2021-07-25T12:27:57Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
M. G. Bernardini (INAF-OAB), P. D'Avanzo (INAF-OAB),
J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), T. Sakamoto (AGU) and T. Sbarrato (INAF-OAB)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 12:00:48 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210725B (trigger=1061554).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 192.932, +17.125 which is 
   RA(J2000) = 12h 51m 44s
   Dec(J2000) = +17d 07' 30"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The immediately-available BAT light curve
(extending to T+8 seconds) shows a complex peak of at least 5 seconds
duration.  The peak count rate was ~1000 counts/sec (15-350 keV), 
at ~2 sec after the trigger. 

The XRT began observing the field at 12:02:18.8 UT, 90.9 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 192.9219,
17.1080 which is equivalent to:
   RA(J2000)  = 12h 51m 41.26s
   Dec(J2000) = +17d 06' 28.9"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 70 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 250 seconds with the U filter starting 91
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 19.2 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 18.0 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. G. Bernardini (grazia.bernardini AT brera.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/)
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