TITLE:   GCN CIRCULAR
NUMBER:  32465
SUBJECT: GRB 220813A: Swift detection of a burst with an optical counterpart
DATE:    22/08/13 19:35:48 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

M. G. Bernardini (INAF-OAB), R. Brivio (INAF-OAB),
M. Ferro (INAF-OAB), J.D. Gropp (PSU), A. Melandri (INAF-OAB),
K. L. Page (U Leicester), D. M. Palmer (LANL),
T. M. Parsotan (GSFC/UMBC/CRESSTII), T. Sbarrato (INAF-OAB) and
M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 19:23:03 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 220813A (trigger=1120270).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 81.501, -32.985 which is 
   RA(J2000) = 05h 26m 00s
   Dec(J2000) = -32d 59' 06"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  The BAT light curve showed a single-peak
structure with a duration of about 30 sec.  The peak count rate
was ~1200 counts/sec (15-350 keV), at ~2 sec after the trigger. 

The XRT began observing the field at 19:24:34.2 UT, 90.6 seconds after
the BAT trigger. Using promptly downlinked data we find a bright,
fading, uncatalogued X-ray source located at RA, Dec 81.53235,
-33.01546 which is equivalent to:
   RA(J2000)  = 05h 26m 07.76s
   Dec(J2000) = -33d 00' 55.7"
with an uncertainty of 3.7 arcseconds (radius, 90% containment). This
location is 144 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 (2.32 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.7
(+3.14/-2.65) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 100 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	05:26:07.57 =  81.53155
  DEC(J2000) = -33:00:58.9  = -33.01636
with a 90%-confidence error radius of about 0.67 arc sec. This position is 4.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
19.61 with a 1-sigma error of about  0.17. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.022. 

Burst Advocate for this burst is M. G. Bernardini (grazia.bernardini AT brera.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/)