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GCN Circular 33505

Subject
GRB 230322B: Swift detection of a burst with optical counterpart
Date
2023-03-22T21:25:32Z (a year ago)
From
Kim Page at U.of Leicester <kimlpage1978@gmail.com>
S. B. Cenko (GSFC), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. P. M. Kuin (UCL-MSSL), S. Laha (GSFC/UMBC),
F. E. Marshall (NASA/GSFC), M. J. Moss (GWU), K. L. Page (U Leicester)
and M. A. Williams (PSU) report on behalf of the Neil Gehrels Swift
Observatory Team:

At 21:10:37 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230322B (trigger=1161127).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 16.565, -47.729 which is 
   RA(J2000) = 01h 06m 16s
   Dec(J2000) = -47d 43' 42"
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 ~30 sec.  The peak count rate
was ~900 counts/sec (15-350 keV), at ~0 sec after the trigger. 

The XRT began observing the field at 21:12:31.7 UT, 113.8 seconds after
the BAT trigger. Using promptly downlinked data we find an uncatalogued
X-ray source with an enhanced position: RA, Dec 16.55359, -47.70106
which is equivalent to:
   RA(J2000)  = 01h 06m 12.86s
   Dec(J2000) = -47d 42' 03.8"
with an uncertainty of 1.9 arcseconds (radius, 90% containment). This
location is 104 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 in excess of the Galactic value (2.48 x
10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5
(+2.99/-2.59) x 10^21 cm^-2 (90% confidence). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 118 seconds after the BAT trigger. There is a candidate afterglow in
the rapidly available 2.7'x2.7' sub-image at
  RA(J2000)  =	01:06:12.85 =  16.55353
  DEC(J2000) = -47:42:02.8  = -47.70079
with a 90%-confidence error radius of about 0.62 arc sec. This position is 0.7
arc sec. from the center of the XRT error circle. The estimated magnitude is
17.60 with a 1-sigma error of about  0.14. No correction has been made for the
expected extinction corresponding to E(B-V) of 0.017. 

Burst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov). 
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/)
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