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GCN Circular 25945

Subject
GRB 191004A: Swift detection of a burst with an optical detection
Date
2019-10-04T18:30:53Z (4 years ago)
From
Scott Barthelmy at NASA/GSFC <scott@milkyway.gsfc.nasa.gov>
S. B. Cenko (GSFC), S. D. Barthelmy (GSFC),
A. P. Beardmore (U Leicester), A. D'Ai (INAF-IASFPA),
V. D'Elia (SSDC), J.D. Gropp (PSU), N. J. Klingler (PSU),
S Laha (CRESST/GSFC/UMD), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU),
K. L. Page (U Leicester), B. Sbarufatti (PSU) and
A. Tohuvavohu (Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 18:07:02 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 191004A (trigger=927825).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 31.721, -36.932 which is 
   RA(J2000)  =  02h 06m 53s
   Dec(J2000) = -36d 55' 53"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a double-peaked
structure with a duration of about 5 sec.  The peak count rate
was ~8000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 18:08:24.2 UT, 81.5 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 31.66950, -36.93368 which is equivalent
to:
   RA(J2000)  = 02h 06m 40.68s
   Dec(J2000) = -36d 56' 01.2"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 148 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. No spectrum from the promptly downlinked
event data is yet available to determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White
filter  starting 84 seconds after the BAT trigger. Results from the
list of sources generated on-board shows an optical source consistent
with the XRT position.  Because there was not automatic aspect
solution for the image,  the precise position and magnitude will be
reported later.  The 2.7'x2.7' sub-image covers none of the XRT error
circle.  No correction has been made for the expected  extinction
corresponding to E(B-V) of 0.01. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko 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/)
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