TITLE:   GCN CIRCULAR
NUMBER:  36208
SUBJECT: GRB 240421B: Swift detection of a burst
DATE:    24/04/21 10:02:18 GMT
FROM:    Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu>


M. A. Williams (PSU), J. J. DeLaunay (PSU), S. Dichiara (PSU),
J.D. Gropp (PSU) and T. Sakamoto (AGU) report on behalf of the Neil
Gehrels Swift Observatory Team:

At 09:42:54 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 240421B (trigger=1223470).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 299.686, -14.879 which is 
   RA(J2000) = 19h 58m 45s
   Dec(J2000) = -14d 52' 42"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a multi-peaked
structure with a duration of about 20 sec.  The peak count rate
was ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 09:45:00.4 UT, 126.3 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 299.70011, -14.86012
which is equivalent to:
   RA(J2000)  = 19h 58m 48.03s
   Dec(J2000) = -14d 51' 36.4"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 83 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density in excess of the Galactic value (1.24 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.8
(+2.62/-2.25) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 129 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
0.183. 

Burst Advocate for this burst is M. A. Williams (mjw6837 AT psu.edu). 
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/)