TITLE:   GCN CIRCULAR
NUMBER:  30568
SUBJECT: GRB 210731A: Swift detection of a burst
DATE:    21/07/31 22:34:42 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

E. Troja (NASA/GSFC/UMCP), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), B. Sbarufatti (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 22:21:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210731A (trigger=1062336).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 300.314, -28.042 which is 
   RA(J2000) = 20h 01m 15s
   Dec(J2000) = -28d 02' 30"
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 40 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 22:24:29.4 UT, 200.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 300.3036, -28.0595 which is equivalent to:
   RA(J2000)  = 20h 01m 12.86s
   Dec(J2000) = -28d 03' 34.2"
with an uncertainty of 5.8 arcseconds (radius, 90% containment). No
event data are yet available to determine the column density using
X-ray spectroscopy. 

The initial flux in the 2.5 s image was 7.96e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of  63 seconds with the White filter
starting 210 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
0.092. 

Burst Advocate for this burst is E. Troja (eleonora.troja AT nasa.gov). 
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/)