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GCN Circular 29794

Subject
GRB 210411C: Swift detection of a burst with an optical counterpart
Date
2021-04-11T15:16:46Z (3 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
T. Sbarrato (INAF-OAB), M. G. Bernardini (INAF-OAB),
P. D'Avanzo (INAF-OAB), J. A. Kennea (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 15:05:51 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210411C (trigger=1042398).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 296.559, -39.373 which is 
   RA(J2000) = 19h 46m 14s
   Dec(J2000) = -39d 22' 21"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~5500 counts/sec (15-350 keV), at ~4 sec after the trigger. 

The XRT began observing the field at 15:06:54.6 UT, 63.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 296.60899, -39.39786 which is
equivalent to:
   RA(J2000)  = 19h 46m 26.16s
   Dec(J2000) = -39d 23' 52.3"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 165 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 (8.10 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.2
(+2.66/-2.35) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of nominal 150.000 seconds with the White
filter starting 66 seconds after the BAT trigger. There is a candidate
afterglow in the list of sources generated on-board at
  RA(J2000)  =	19:46:26.83 = 296.61178
  DEC(J2000) = -39:23:51.9  = -39.39774
with a 90%-confidence error radius of about 1.10 arc sec. This position is 7.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.99. No correction has been made for the expected extinction corresponding to
E(B-V) of 0.080. 

Burst Advocate for this burst is T. Sbarrato (tullia.sbarrato 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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