[vsnet-grb-info 28549] GRB 210818A: Swift detection of a burst with an optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30652
SUBJECT: GRB 210818A: Swift detection of a burst with an optical counterpart
DATE:    21/08/18 01:26:40 GMT
FROM:    Kim Page at U.of Leicester  <klp5 at leicester.ac.uk>

A. P. Beardmore (U Leicester), J. A. Kennea (PSU),
A. Y. Lien (GSFC/UMBC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 01:02:08 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210818A (trigger=1069169).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 149.131, -87.031 which is 
   RA(J2000) = 09h 56m 32s
   Dec(J2000) = -87d 01' 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 60 sec.  The peak count rate
was ~28000 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 01:03:06.9 UT, 59.0 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source with an enhanced position: RA, Dec 148.8666,
-87.0253 which is equivalent to:
   RA(J2000)  = 09h 55m 27.99s
   Dec(J2000) = -87d 01' 31.0"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 53 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 in excess of the Galactic value (1.74 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 4.7
(+2.38/-2.11) x 10^21 cm^-2 (90% confidence). 

The initial flux in the 2.5 s image was 1.40e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 66 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	09:55:28.15 = 148.86729
  DEC(J2000) = -87:01:28.8  = -87.02467
with a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.93 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.213. 

Although this was originally reported in the GCN Notices as a known source
we believe that this is a GRB. 

Burst Advocate for this burst is A. P. Beardmore (apb 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/)