TITLE:   GCN CIRCULAR
NUMBER:  31960
SUBJECT: GRB 220427A: Swift detection of a burst with an optical counterpart
DATE:    22/04/27 21:26:22 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), C. Gronwall (PSU), J.D. Gropp (PSU),
J. A. Kennea (PSU), S. Laha (GSFC/UMBC), D. M. Palmer (LANL) and
A. Tohuvavohu (U Toronto) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:00:34 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220427A (trigger=1104343).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 275.869, -56.253 which is 
   RA(J2000) = 18h 23m 29s
   Dec(J2000) = -56d 15' 09"
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 80 sec.  The peak count rate
was ~4000 counts/sec (15-350 keV), at ~10 sec after the trigger. 

The XRT began observing the field at 21:03:33.0 UT, 178.1 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source located at RA, Dec 275.88860, -56.25508 which is
equivalent to:
   RA(J2000)  = 18h 23m 33.26s
   Dec(J2000) = -56d 15' 18.3"
with an uncertainty of 3.5 arcseconds (radius, 90% containment). This
location is 39 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 (1.02 x
10^21 cm^-2, Willingale et al. 2013), with an excess column of 2.9
(+3.18/-2.73) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White 
filter  starting 181 seconds after the BAT trigger. There is a 
candidate afterglow in  the rapidly available 2.7'x2.7' sub-image at  
RA(J2000) = 18:23:33.32 = 275.88882    
DEC(J2000) = -56:15:18.3 = -56.25507    
with a 90%-confidence error radius of about 0.61 arc sec. 
This position is 1.4  arc sec. from the center of the XRT error
circle. The estimated magnitude is  17.23 with a 1-sigma error of
about 0.14. No correction has been made for the  expected extinction
corresponding to E(B-V) of 0.097. 

The original BAT Position Notice found an approximate coincidence
(~10 arcmin) with the position of IGR J18244-5622 (which in turn 
is coincident with the Sey2 galaxy IC 4709 at z=0.017). 
However, the position of the X-ray and Optical counterpart 
excludes this source and galaxy, indicating that this is a new GRB. 

Burst Advocate for this burst is A. D'Ai (antonino.dai 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/)