TITLE:   GCN CIRCULAR
NUMBER:  31820
SUBJECT: GRB 220403B: Swift detection of a burst with an optical counterpart
DATE:    22/04/03 20:54:46 GMT
FROM:    Hans Krimm at NSF/NASA-GSFC  <hkrimm@nsf.gov>

N. J. Klingler (GSFC/UMBC/CRESSTII), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), S. Dichiara (PSU), J.D. Gropp (PSU),
H. A. Krimm (NSF), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
T. M. Parsotan (GSFC/UMBC/CRESSTII) and T. Sakamoto (AGU) report on
behalf of the Neil Gehrels Swift Observatory Team:

At 20:42:42 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220403B (trigger=1101053).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 191.025, +89.170 which is 
   RA(J2000) = 12h 44m 06s
   Dec(J2000) = +89d 10' 13"
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 30 sec and a possible precursor just before. 
The peak count rate was ~2800 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 20:44:18.9 UT, 96.3 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
uncatalogued X-ray source located at RA, Dec 191.50031, 89.18362 which
is equivalent to:
   RA(J2000)  = 12h 46m 00.07s
   Dec(J2000) = +89d 11' 01.0"
with an uncertainty of 3.9 arcseconds (radius, 90% containment). This
location is 54 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. 

A power-law fit to a spectrum formed from promptly downlinked event
data gives a column density consistent with the Galactic value of 1.20
x 10^21 cm^-2 (Willingale et al. 2013). 

The initial flux in the 2.5 s image was 4.65e-10 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 105 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	12:45:53.30 = 191.47207
  DEC(J2000) = +89:11:06.6  =  89.18518
with a 90%-confidence error radius of about 0.67 arc sec. This position is 6.0
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.80 with a 1-sigma error of about  0.17. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.208. 

Burst Advocate for this burst is N. J. Klingler (noelklin AT umbc.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/)