TITLE: GCN CIRCULAR NUMBER: 41462 SUBJECT: GRB 250821A: Swift detection of a burst DATE: 25/08/21 11:00:36 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> S. Dichiara (PSU), R. A. J. Eyles-Ferris (U Leicester), R. Gupta (NASA GSFC), K. L. Page (U Leicester), B. Sbarufatti (INAF-OAB) and M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift Observatory Team: At 10:46:17 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250821A (trigger=1344131). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 358.412, -28.618 which is RA(J2000) = 23h 53m 39s Dec(J2000) = -28d 37' 05" 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 20 sec. The peak count rate was ~2476 counts/sec (15-350 keV), at ~0 sec after the trigger. The XRT began observing the field at 10:48:40.1 UT, 142.8 seconds after the BAT trigger. Using promptly downlinked data we find an uncatalogued X-ray source located at RA, Dec 358.43969, -28.65380 which is equivalent to: RA(J2000) = 23h 53m 45.53s Dec(J2000) = -28d 39' 13.7" with an uncertainty of 4.8 arcseconds (radius, 90% containment). This location is 155 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. We cannot determine whether the source is fading at the present time. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.65 x 10^20 cm^-2 (Willingale et al. 2013). UVOT took a finding chart exposure of 150 seconds with the White filter starting 147 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 none of the XRT error circle. 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 mag. No correction has been made for the expected extinction corresponding to E(B-V) of 0.015. Burst Advocate for this burst is S. Dichiara (sbd5667 AT 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/41462. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...