TITLE:   GCN CIRCULAR
NUMBER:  30897
SUBJECT: GRB 210930A: Swift detection of a burst
DATE:    21/09/30 02:58:45 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <siegel@swift.psu.edu>

V. D'Elia (SSDC & INAF-OAR), E. Ambrosi (INAF-IASFPA),
S. Laha (GSFC/UMBC/CRESST), A. Y. Lien (U Tampa),
F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL), M. H. Siegel (PSU)
and A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels
Swift Observatory Team:

At 02:46:52 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 210930A (trigger=1075565).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 197.435, +48.623 which is 
   RA(J2000) = 13h 09m 44s
   Dec(J2000) = +48d 37' 22"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peaked
structure with a duration of about 15 sec.  The peak count rate
was ~800 counts/sec (15-350 keV), at ~8 sec after the trigger. 

The XRT began observing the field at 02:49:12.3 UT, 140.1 seconds after
the BAT trigger. Using promptly downlinked data we find a fading,
uncatalogued X-ray source located at RA, Dec 197.40185, 48.62853 which
is equivalent to:
   RA(J2000)  = 13h 09m 36.44s
   Dec(J2000) = +48d 37' 42.7"
with an uncertainty of 3.8 arcseconds (radius, 90% containment). This
location is 81 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 (9.55 x
10^19 cm^-2, Willingale et al. 2013), with an excess column of 6.4
(+3.87/-3.20) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 143 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 none of
the XRT error circle. 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 0.010. 

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/)