TITLE: GCN CIRCULAR NUMBER: 10654 SUBJECT: Trigger 420256: Swift detection of a possible GRB DATE: 10/04/23 03:09:51 GMT FROM: David Palmer at LANL E. A. Hoversten (PSU), S. D. Barthelmy (GSFC), P. A. Evans (U Leicester), J. M. Gelbord (PSU), D. Grupe (PSU), S. T. Holland (CRESST/USRA/GSFC), S. Immler (CRESST/GSFC/UMD), W.B Landsman (GSFC), C. B. Markwardt (CRESST/GSFC/UMD), D. M. Palmer (LANL), M. H. Siegel (PSU), M. C. Stroh (PSU), T. N. Ukwatta (GSFC/GWU) and L. Vetere (PSU) report on behalf of the Swift Team: At 02:44:02 UT, the Swift Burst Alert Telescope (BAT) detected trigger 420256. Swift slewed immediately to the location. The BAT on-board calculated location is RA, Dec 271.841, +27.172 which is RA(J2000) = 18h 07m 22s Dec(J2000) = +27d 10' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for an image trigger, there is no obvious variation in the immediately-available lightcurve. The XRT began observing the field at 02:46:32.1 UT, 149.9 seconds after the BAT trigger. No source was detected in the promptly available XRT 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 153 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 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. The list of sources is typically complete to about 18 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.11. Due to the marginal significance (7.02 sigma) of the initial detection by BAT, and the lack of a source detection by XRT, we cannot confirm that this is a true astrophysical source. Final determination of the reality of this detection will require analysis of the Malindi data. Burst Advocate for this burst is E. A. Hoversten (hoversten AT astro.psu.edu). 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/too.html.)