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GCN Circular 16928

Subject
GRB 141020A: Swift detection of a burst with an optical counterpart
Date
2014-10-20T08:01:15Z (10 years ago)
From
Hans Krimm at NASA-GSFC <hans.krimm@nasa.gov>
S. B. Cenko (GSFC), A. P. Beardmore (U Leicester),
M. M. Chester (PSU), H. A. Krimm (CRESST/GSFC/USRA),
P. T. O'Brien (U Leicester) and K. L. Page (U Leicester) report on
behalf of the Swift Team:

At 07:48:39 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 141020A (trigger=615873).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 225.009, +55.334 which is 
   RA(J2000) = 15h 00m 02s
   Dec(J2000) = +55d 20' 02"
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 30 sec.  The peak count rate
was ~1700 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 07:50:06.5 UT, 87.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 224.99514, 55.31391 which is equivalent
to:
   RA(J2000)  = 14h 59m 58.83s
   Dec(J2000) = +55d 18' 50.1"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 77 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 consistent with the Galactic value of 1.25
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 91 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	14:59:59.15 = 224.99647
  DEC(J2000) = +55:18:46.8  =  55.31299
with a 90%-confidence error radius of about 0.75 arc sec. This position is 4.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.73 with a 1-sigma error of about  0.14. 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/too.html.)
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