GCN Circular 29849

M. J. Moss (GWU), S. D. Barthelmy (GSFC), C. Gronwall (PSU),
J.D. Gropp (PSU), J. A. Kennea (PSU), S. Laha (GSFC/UMBC/CRESST),
A. Y. Lien (GSFC/UMBC), F. E. Marshall (NASA/GSFC),
K. L. Page (U Leicester), D. M. Palmer (LANL), B. Sbarufatti (PSU) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 00:27:30 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210421A (trigger=1044426).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 104.908, +4.934, which is 
   RA(J2000)  =  06h 59m 38s
   Dec(J2000) = +04d 56' 03"
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 40 sec.  The peak count rate
was ~1500 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 00:28:58.1 UT, 87.8 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 104.87569, 4.92168 which
is equivalent to:
   RA(J2000)  = 06h 59m 30.17s
   Dec(J2000) = +04d 55' 18.0"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 124 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
https://www.swift.ac.uk/sper.  We cannot determine whether the source
is fading at the present time. No spectrum from the promptly downlinked
event data is yet available to determine the column density. 

The initial flux in the 0.1 s image was 1.51e-08 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 96 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 large, but uncertain, extinction expected. 

Burst Advocate for this burst is M. J. Moss (mikejmoss3 AT gmail.com). 
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/)