TITLE:   GCN CIRCULAR
NUMBER:  33260
SUBJECT: GRB 230204A: Swift detection of a possible burst (or possible transient Swift J1552.8-5055)
DATE:    23/02/04 03:45:02 GMT
FROM:    David Palmer at LANL  <palmer@lanl.gov>

A. D'Ai (INAF-IASFPA), E. Ambrosi (INAF-IASFPA),
V. D'Elia (SSDC & INAF-OAR), J. A. Kennea (PSU), A. Y. Lien (U Tampa),
A. Melandri (INAF-OAB), D. M. Palmer (LANL), B. Sbarufatti (INAF-OAB),
M. H. Siegel (PSU), A. Tohuvavohu (U Toronto) and M. A. Williams (PSU)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 02:56:57 UT, the Swift Burst Alert Telescope (BAT) triggered and
located GRB 230204A (trigger=1152509).  Swift slewed immediately to the burst. 
The BAT on-board calculated location is 
RA, Dec 238.164d, -50.858d which is 
   RA(J2000) = 15h 52m 39s
   Dec(J2000) = -50d 51' 27"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  Due to a telemetry gap, the BAT lightcurve
before T+8s is not immediately available, and no obvious variation is
visible in the remainder of the lightcurve, as is typical for an
image trigger. 

The XRT began observing the field at 02:59:24.1 UT, 146.8 seconds after
the BAT trigger. XRT found a bright, uncatalogued X-ray source located
at RA, Dec 238.1950, -50.9167 which is equivalent to:
   RA(J2000)  = 15h 52m 46.80s
   Dec(J2000) = -50d 55' 00.1"
with an uncertainty of 5.0 arcseconds (radius, 90% containment). 

The initial flux in the 2.5 s image was 2.77e-09 erg cm^-2 s^-1 (0.2-10
keV). 

UVOT took a finding chart exposure of 150 seconds with the White filter
starting 156 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 large, but
uncertain, extinction expected. 

We note that this source is near the Galactic plane (lat=2.38 deg)
and was detected as a 64 s image trigger, which raises the possibility
that this is a Galactic transient.  We would name this source
Swift J1552.8-5055 if it is not a GRB. 

Further analysis of the nature of this source will require the
full downlinked dataset. 

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/)