[vsnet-grb-info 20752] GRB 180626A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  22850
SUBJECT: GRB 180626A: Swift detection of a burst with an optical counterpart
DATE:    18/06/26 08:34:26 GMT
FROM:    Kim Page at U.of Leicester  <klp5 at leicester.ac.uk>

P. A. Evans (U Leicester), A. P. Beardmore (U Leicester),
J.D. Gropp (PSU), H. A. Krimm (NSF/USRA), A. Y. Lien (GSFC/UMBC) and
K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:21:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 180626A (trigger=844615).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 243.567, +14.764 which is 
   RA(J2000) = 16h 14m 16s
   Dec(J2000) = +14d 45' 52"
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 50 sec.  The peak count rate
was ~3400 counts/sec (15-350 keV), at ~-2 sec after the trigger. 

The XRT began observing the field at 08:23:16.9 UT, 120.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 243.57544, 14.75598 which is equivalent
to:
   RA(J2000)  = 16h 14m 18.11s
   Dec(J2000) = +14d 45' 21.5"
with an uncertainty of 3.6 arcseconds (radius, 90% containment). This
location is 41 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 in excess of the Galactic value (3.47 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.2
(+2.28/-2.05) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 123 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	16:14:18.12 = 243.57551
  DEC(J2000) = +14:45:25.5  =  14.75707
with a 90%-confidence error radius of about 0.81 arc sec. This position is 5.1
arc sec. from the center of the XRT error circle. The estimated magnitude is
20.71 with a 1-sigma error of about  0.23. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.04. 

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.)