[vsnet-grb-info 26596] GRB 201020A: Swift detection of a burst with an optical counterpart.

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  28696
SUBJECT: GRB 201020A: Swift detection of a burst with an optical counterpart. 
DATE:    20/10/20 05:59:53 GMT
FROM:    Kim Page at U.of Leicester  <klp5 at leicester.ac.uk>

E. Ambrosi (INAF-IASFPA), A. D'Ai (INAF-IASFPA), V. D'Elia (SSDC),
N. J. Klingler (PSU), H. A. Krimm (NSF), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), T. Sakamoto (AGU), B. Sbarufatti (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 05:47:26 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 201020A (trigger=1000926).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 261.246, +31.425 which is 
   RA(J2000) = 17h 24m 59s
   Dec(J2000) = +31d 25' 30"
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 20 sec.  The peak count rate
was ~1800 counts/sec (15-350 keV), at ~17 sec after the trigger. 

The XRT began observing the field at 05:49:47.6 UT, 141.5 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source with an enhanced position: RA, Dec 261.2279,
31.4288 which is equivalent to:
   RA(J2000)  = 17h 24m 54.71s
   Dec(J2000) = +31d 25' 43.8"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 57 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 consistent with the Galactic value of 3.70
x 10^20 cm^-2 (Willingale et al. 2013). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 144 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	17:24:54.67 = 261.22779
  DEC(J2000) = +31:25:42.0  =  31.42834
with a 90%-confidence error radius of about 0.62 arc sec. This position is 5.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.14 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi 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/)