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GCN Circular 30475

Subject
GRB 210722A: Swift detection of a burst with a bright optical counterpart
Date
2021-07-22T21:13:43Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
P. D'Avanzo (INAF-OAB), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB), M. J. Moss (GWU),
D. M. Palmer (LANL) and A. Tohuvavohu (U Toronto) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 20:54:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210722A (trigger=1061223).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 27.023, -6.354 which is 
   RA(J2000) = 01h 48m 06s
   Dec(J2000) = -06d 21' 14"
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 30 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

Using promptly downlinked XRT data we find an uncatalogued X-ray source located
at RA, Dec 27.028, -6.345 which is equivalent to:
   RA(J2000)  = +01h 48m 06.84
   Dec(J2000) = -06d 20' 42.7"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 94 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
RA(J2000) = 01:48:07.27 = 27.03028
DEC(J2000) = -06:20:49.8 = -6.34718
with a 90%-confidence error radius of about 0.74 arc sec. This position is 9.6
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.37 with a 1-sigma error of about 0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.027. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo 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/)
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