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GCN Circular 30497

Subject
GRB 210724A: Swift detection of a burst
Date
2021-07-24T20:37:28Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
P. D'Avanzo (INAF-OAB), M. G. Bernardini (INAF-OAB),
J. A. Kennea (PSU), N. J. Klingler (GSFC/UMBC/CRESSTII),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
A. Melandri (INAF-OAB), D. M. Palmer (LANL) and T. Sbarrato (INAF-OAB)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 20:14:09 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210724A (trigger=1061482).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 227.407d, -6.274d which is 
   RA(J2000) = 15h 09m 38s
   Dec(J2000) = -06d 16' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty). Due to a telemetry outage the BAT lightcurve is
not immediately available.  However, the trigger information indicates
a duration of at least 4 seconds. 

The XRT began observing the field at 20:17:09.0 UT, 179.8 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 227.4118,
-6.2911 which is equivalent to:
   RA(J2000)  = 15h 09m 38.83s
   Dec(J2000) = -06d 17' 27.8"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). 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 consistent with the Galactic value of 8.23
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 124 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
0.097. 

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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