GCN Circular 29610

E. Ambrosi (INAF-IASFPA), S. D. Barthelmy (GSFC),
A. D'Ai (INAF-IASFPA), N. J. Klingler (PSU), K. L. Page (U Leicester)
and T. Sakamoto (AGU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 08:42:38 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210307A (trigger=1036139).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 119.190, -66.963 which is 
   RA(J2000) = 07h 56m 46s
   Dec(J2000) = -66d 57' 45"
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 120 sec.  The peak count rate
was ~1300 counts/sec (15-350 keV), at ~3 sec after the trigger. 

The XRT began observing the field at 96.2 s seconds after
the BAT trigger. Using promptly downlinked data we find an
uncatalogued X-ray source with a position: RA, Dec 119.239583, 
-66.969694 which is equivalent to: 
   RA(J2000)  = 07 56 57.5
   Dec(J2000) = -66 58 10.9 
with an uncertainty of 4.7 arcseconds (radius, 90% containment). This
location is 73.8 arcsec 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 100 seconds after the BAT trigger. No credible afterglow candidate has
been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of
the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. 
The 8'x8' region for the list of sources generated on-board covers 100% of the
XRT error circle. The list of sources is typically complete to about 18 mag. No
correction has been made for the expected extinction corresponding to E(B-V) of
0.138. 

This source lies within the current (Sector 35) field-of-view of TESS camera 4. 

Burst Advocate for this burst is E. Ambrosi (elena.ambrosi 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/)