[vsnet-grb-info 28067] GRB 210610B: Swift detection of a burst with bright optical afterglow

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30170
SUBJECT: GRB 210610B: Swift detection of a burst with bright optical afterglow
DATE:    21/06/10 20:06:36 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

K. L. Page (U Leicester), J.D. Gropp (PSU), J. A. Kennea (PSU),
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 19:51:27 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210610B (trigger=1054681).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 243.939, +14.391 which is 
   RA(J2000) = 16h 15m 45s
   Dec(J2000) = +14d 23' 29"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a comlex
structure with a duration of about 100 sec.  The peak count rate
was ~11000 counts/sec (15-350 keV), at ~8 sec after the trigger. 

The XRT began observing the field at 19:52:51.4 UT, 83.9 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 243.9180, 14.3977 which is equivalent to:
   RA(J2000)  = 16h 15m 40.32s
   Dec(J2000) = +14d 23' 51.7"
with an uncertainty of 6.7 arcseconds (radius, 90% containment). This
location is 77 arcseconds from the BAT onboard position, within the BAT
error circle. No event data are yet available to determine the column
density using X-ray spectroscopy. 

The initial flux in the 0.1 s image was 1.07e-08 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)  =	16:15:40.41 = 243.91836
  DEC(J2000) = +14:23:56.7  =  14.39909
with a 90%-confidence error radius of about 0.61 arc sec. This position is 5.2
arc sec. from the center of the XRT error circle. The estimated magnitude is
13.70 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.044. 

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/)