TITLE:   GCN CIRCULAR
NUMBER:  29619
SUBJECT: GRB 210308A: Swift detection of a burst with an optical counterpart
DATE:    21/03/08 06:49:21 GMT
FROM:    Scott Barthelmy at NASA/GSFC  <scott@milkyway.gsfc.nasa.gov>

V. D'Elia (SSDC), E. Ambrosi (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), N. J. Klingler (PSU), A. Y. Lien (GSFC/UMBC),
K. L. Page (U Leicester), T. Sakamoto (AGU), M. H. Siegel (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 06:37:56 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210308A (trigger=1036227).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 67.088, +37.411 which is 
   RA(J2000)  =  04h 28m 21s
   Dec(J2000) = +37d 24' 39"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve shows a few overlapping peaks
with a duration of about 15 sec.  The peak count rate
was ~6000 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 06:39:21.81 UT, 85.0 seconds after
the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source with a position: RA, Dec 67.076417, 
37.435310 which is equivalent to:
   RA(J2000)  = 04h 28m 18.34s
   Dec(J2000) = +37d 26' 07.11"
with an uncertainty of 4.3 arcseconds (radius, 90% containment). This
location is 94 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 http://www.swift.ac.uk/sper. 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 94 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	04:28:17.73 =  67.07388
  DEC(J2000) = +37:26:02.0  =  37.43388
with a 90%-confidence error radius of about 0.63 arc sec. This position is 8.5
arc sec. from the center of the XRT error circle. The estimated magnitude is
18.57 with a 1-sigma error of about  0.15. No correction has been made for the
expected extinction corresponding to E(B-V) of 1.295. 

Burst Advocate for this burst is V. D'Elia (valerio.delia AT ssdc.asi.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/)