TITLE:   GCN CIRCULAR
NUMBER:  33677
SUBJECT: Swift Trigger 1165354 is probably not a GRB
DATE:    23/04/23 19:40:27 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>


R. Brivio (INAF-OAB), J.D. Gropp (PSU), J. A. Kennea (PSU),
N. J. Klingler (GSFC/UMBC/CRESSTII), N. P. M. Kuin (UCL-MSSL),
S. Laha (GSFC/UMBC), F. E. Marshall (NASA/GSFC), D. M. Palmer (LANL),
C. Salvaggio (INAF-OAB) and T. Sbarrato (INAF-OAB) report on behalf of
the Neil Gehrels Swift Observatory Team:

At 19:07:55 UT, the Swift Burst Alert Telescope (BAT) triggered and located
a marginal image peak, which is possibly GRB 230423A (trigger=1165354). 
Swift slewed immediately to the image location. 
The BAT on-board calculated location is 
RA, Dec 122.605, -21.920 which is 
   RA(J2000) = 08h 10m 25s
   Dec(J2000) = -21d 55' 12"
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 ~600 counts/sec (15-350 keV), at ~1 sec after the trigger. 

The XRT began observing the field at 19:09:36.5 UT, 101.1 seconds after
the BAT trigger. No source was detected in 777 s of promptly downlinked
data. We are waiting for the full dataset to detect and localise the
XRT counterpart. 

UVOT took a finding chart exposure of 150 seconds with the White 
filter  starting 885 seconds after the BAT trigger. No credible 
afterglow candidate has  been found in the initial data products. The 
2.7'x2.7' covers 25% of the BAT error circle. The typical 3-sigma
upper limit has been about 19.6 mag. The 8'x8'  region for  the list
of sources generated on-board covers 100% of the BAT error  circle. No
correction has been made for the expected extinction   corresponding
to E(B-V) of 0.136. 

Due to the marginal significance (7.3 sigma) of this detection in BAT
and the lack of an observed XRT counterpart, we cannot confirm
that this is a real GRB.  However, the XRT observation was
interrupted by entry into the South Atlantic Anomaly (SAA)
so the bulk of the XRT exposure time may have been after
any source could have faded significantly.  For this reason,
we will not be able to verify or refute the reality of this
detection until the full dataset is downloaded.