[vsnet-grb-info 20709] GRB 180620B: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22807
SUBJECT: GRB 180620B: Swift detection of a burst with an optical counterpart
DATE:    18/06/20 16:05:27 GMT
FROM:    Kim Page at U.of Leicester  <klp5 at leicester.ac.uk>

P. A. Evans (U Leicester), A. A. Breeveld (UCL-MSSL), A. Deich (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), H. A. Krimm (NSF/USRA),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), M. H. Siegel (PSU) and A. Tohuvavohu (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 15:50:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180620B (trigger=843211).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 357.517, -57.957 which is 
   RA(J2000) = 23h 50m 04s
   Dec(J2000) = -57d 57' 25"
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 30 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 15:52:00.3 UT, 83.2 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 357.52114, -57.96306 which
is equivalent to:
   RA(J2000)  = 23h 50m 05.07s
   Dec(J2000) = -57d 57' 47.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 23 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.39
x 10^20 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 7.04e-09 erg cm^-2 s^-1 (0.2-10
keV). 

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)  =	23:50:05.10 = 357.52126
  DEC(J2000) = -57:57:44.1  = -57.96224
with a 90%-confidence error radius of about 0.75 arc sec. This position is 1.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.98 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 P. A. Evans (pae9 AT star.le.ac.uk). 
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.)