TITLE: GCN CIRCULAR NUMBER: 41511 SUBJECT: GRB 250823A: Swift detection of a burst DATE: 25/08/23 20:31:35 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL), S. Lanava (PSU), K. L. Page (U Leicester), D. M. Palmer (LANL) and B. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift Observatory Team: At 20:10:07 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250823A (trigger=1344586). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 297.521, -24.976 which is RA(J2000) = 19h 50m 05s Dec(J2000) = -24d 58' 33" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a complex structure with a duration of 200 sec. The peak count rate was ~1600 counts/sec (15-350 keV), at ~4 sec after the trigger. The XRT began observing the field at 20:17:28.0 UT, 440.5 seconds after the BAT trigger. Using promptly downlinked data we find a fading, uncatalogued X-ray source with an enhanced position: RA, Dec 297.47788, -24.99582 which is equivalent to: RA(J2000) = 19h 49m 54.69s Dec(J2000) = -24d 59' 45.0" with an uncertainty of 2.1 arcseconds (radius, 90% containment). This location is 157 arcseconds from the BAT onboard position, within the BAT error circle. This position may be improved as more data are received; the latest position is available at https://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density in excess of the Galactic value (1.12 x 10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.9 (+3.01/-2.59) x 10^21 cm^-2 (90% confidence). UVOT took a finding chart exposure of 250 seconds with the U filter starting 338 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 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. 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 18.0 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.111. Some data, such as earlier XRT observations, are not available due to a telemetry gap, and will be produced on the full ground download. Burst Advocate for this burst is M. H. Siegel (siegel AT swift.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/) View this GCN Circular online at https://gcn.nasa.gov/circulars/41511. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...