[vsnet-grb-info 28158] GRB 210619B: Swift detection of a bright burst and optical counterpart

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  30261
SUBJECT: GRB 210619B: Swift detection of a bright burst and optical counterpart
DATE:    21/06/20 00:12:00 GMT
FROM:    David Palmer at LANL  <palmer at lanl.gov>

P. D'Avanzo (INAF-OAB), M. G. Bernardini (INAF-OAB),
A. Y. Lien (GSFC/UMBC), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), D. M. Palmer (LANL) and
T. Sbarrato (INAF-OAB) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 23:59:25 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210619B (trigger=1056757).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 319.713, +33.860 which is 
   RA(J2000) = 21h 18m 51s
   Dec(J2000) = +33d 51' 35"
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 ~100,000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 00:04:53.4 UT, 328.1 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 319.7161, 33.8495 which is equivalent to:
   RA(J2000)  = 21h 18m 51.86s
   Dec(J2000) = +33d 50' 58.2"
with an uncertainty of 11.1 arcseconds (radius, 90% containment). This
location is 38 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 2.5 s image was 4.27e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 135 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	21:18:52.39 = 319.71831
  DEC(J2000) = +33:51:01.6  =  33.85044
with a 90%-confidence error radius of about 0.61 arc sec. This position is 7.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
14.71 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.173. 

Burst Advocate for this burst is P. D'Avanzo (paolo.davanzo AT inaf.it). 
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/)