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GCN Circular 25988

Subject
GRB 191011A: Swift detection of a burst with an optical counterpart
Date
2019-10-11T04:46:45Z (4 years ago)
From
David Palmer at LANL <palmer@lanl.gov>
S. L Laha (GSFC/UMBC/CRESST), C. Gronwall (PSU), J.D. Gropp (PSU),
N. P. M. Kuin (UCL-MSSL), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
E. Troja (NASA/GSFC/UMCP) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 04:35:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191011A (trigger=928924).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 44.724, -27.845 which is 
   RA(J2000) = 02h 58m 54s
   Dec(J2000) = -27d 50' 40"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a complex peak
structure with a duration of about 20 sec.  The peak count rate
was ~3000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 04:37:11.1 UT, 74.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 44.72679, -27.84594 which is equivalent
to:
   RA(J2000)  = 02h 58m 54.43s
   Dec(J2000) = -27d 50' 45.4"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 9.5 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
http://www.swift.ac.uk/sper.  We cannot determine whether the source is
fading at the present time. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.54 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.8
(+3.35/-2.89) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 77 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	02:58:54.80 =  44.72832
  DEC(J2000) = -27:50:43.5  = -27.84541
with a 90%-confidence error radius of about 0.83 arc sec. This position is 6.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.20 with a 1-sigma error of about  0.19. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.02. 

Burst Advocate for this burst is S. L Laha (sib.laha AT gmail.com). 
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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