TITLE:   GCN CIRCULAR
NUMBER:  37623
SUBJECT: Swift Trigger 1256950 is probably not a GRB
DATE:    24/09/29 04:57:23 GMT
FROM:    David Palmer at LANL <palmer@lanl.gov>

R. Gupta (NASA GSFC), K. L. Page (U Leicester) and D. M. Palmer (LANL)
report on behalf of the Neil Gehrels Swift Observatory Team:

At 04:14:16 UT, the Swift Burst Alert Telescope (BAT) triggered and
located an image peak (trigger=1256950).  Swift slewed immediately to
the peak. Due to a telemetry gap, the peak location was not immediately
transmitted to the ground.  However a detector plane map was sent to the
ground indicating a marginal-significance peak.
The BAT ground-calculated location is 
RA, Dec 35.089, -54.581 which is 
   RA(J2000) = 02h 20m 21s
   Dec(J2000) = -54d 34' 50"
with an uncertainty of 3 arcmin (radius, 90% containment, including 
systematic uncertainty).  No BAT lightcurve is immediately
available.

The XRT began observing the field at 04:15:43.0 UT, 87 seconds after the BAT
trigger. No source was detected in 433 s of promptly downlinked data. We are
waiting for the full dataset to detect and localise the XRT counterpart.

The lack of immediate data makes it hard to evaluate this trigger.
However, the geographic location of the spacecraft at the time of
the burst (near the South Atlantic Anomaly), the low significance
(6.4 sigma) of the peak in the ground-calculated image,
and the lack a detected XRT counterpart argue against the
reality of this as an astrophysical source.  Therefore we
conclude that this is probably not a GRB.  A final determination
will require the full downlinked dataset.