TITLE:   GCN CIRCULAR
NUMBER:  30160
SUBJECT: GRB 210610A: Swift detection of a burst with an optical counterpart
DATE:    21/06/10 15:26:44 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

K. L. Page (U Leicester), M. A. Baer (PSU), J.D. Gropp (PSU),
N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL) and M. H. Siegel (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 15:03:43 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210610A (trigger=1054627).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 204.285, +14.480 which is 
   RA(J2000) = 13h 37m 08s
   Dec(J2000) = +14d 28' 49"
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 15 sec.  The peak count rate
was ~2000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 15:05:13.3 UT, 90.0 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 204.2823, 14.4652 which
is equivalent to:
   RA(J2000)  = 13h 37m 07.76s
   Dec(J2000) = +14d 27' 54.7"
with an uncertainty of 2.2 arcseconds (radius, 90% containment). This
location is 54 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. No
spectrum from the promptly downlinked event data is yet available to
determine the column density. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 93 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	13:37:07.62 = 204.28174
  DEC(J2000) = +14:27:55.0  =  14.46528
with a 90%-confidence error radius of about 0.61 arc sec. This position is 2.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.12 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.032. 

Burst Advocate for this burst is K. L. Page (klp5 AT leicester.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/)