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. View this GCN Circular online at https://gcn.nasa.gov/circulars/37623. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...