[vsnet-grb-info 28248] GRB 210702A: Swift detection of a burst with a bright optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30351
SUBJECT: GRB 210702A: Swift detection of a burst with a bright optical counterpart
DATE:    21/07/02 19:31:14 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

A. Y. Lien (GSFC/UMBC), S. D. Barthelmy (GSFC), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), S. Laha (GSFC/UMBC/CRESST),
F. E. Marshall (NASA/GSFC), K. L. Page (U Leicester) and
D. M. Palmer (LANL) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 19:07:13 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210702A (trigger=1058804).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 168.570, -36.741 which is 
   RA(J2000) = 11h 14m 17s
   Dec(J2000) = -36d 44' 28"
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 100 sec.  The peak count rate
was ~10,000 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 19:08:49.2 UT, 95.5 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 168.5813, -36.7474 which is equivalent to:
   RA(J2000)  = 11h 14m 19.51s
   Dec(J2000) = -36d 44' 50.6"
with an uncertainty of 11.0 arcseconds (radius, 90% containment). No
event data are yet available to determine the column density using
X-ray spectroscopy. 

The initial flux in the 2.5 s image was 1.00e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of  45 seconds with the U filter starting
315 seconds after the BAT trigger. There is a candidate afterglow in the
rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	11:14:18.83 = 168.57846
  DEC(J2000) = -36:44:48.8  = -36.74690
with a 90%-confidence error radius of about 0.61 arc sec. This position is 8.4
arc sec. from the center of the XRT error circle. The estimated magnitude is
11.72 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.121. 

Burst Advocate for this burst is A. Y. Lien (amy.y.lien AT nasa.gov). 
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/)