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GCN Circular 30960

Subject
GRB 211023B: Swift detection of a possibly short burst with an optical counterpart
Date
2021-10-23T21:16:59Z (2 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
S. Dichiara (NASA/GSFC/UMCP), J.D. Gropp (PSU), J. A. Kennea (PSU),
H. A. Krimm (NSF), N. P. M. Kuin (UCL-MSSL), A. Y. Lien (U Tampa),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
D. M. Palmer (LANL), B. Sbarufatti (PSU), M. H. Siegel (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:05:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 211023B (trigger=1080859).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 170.302, +39.137 which is 
   RA(J2000) = 11h 21m 12s
   Dec(J2000) = +39d 08' 14"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 2 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 21:07:27.3 UT, 95.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 170.31048, 39.13595 which is equivalent
to:
   RA(J2000)  = 11h 21m 14.52s
   Dec(J2000) = +39d 08' 09.4"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 23 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. 

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 3.5
(+2.51/-2.21) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 97 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:21:14.31 = 170.30963
  DEC(J2000) = +39:08:08.8  =  39.13578
with a 90%-confidence error radius of about 0.79 arc sec. This position is 3.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.32 with a 1-sigma error of about  0.16. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.024. 

We note that this is Swift's 1500th GRB. 

Burst Advocate for this burst is S. Dichiara (dichiara AT umd.edu). 
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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