TITLE: GCN CIRCULAR NUMBER: 41898 SUBJECT: GRB 250920B: Swift detection of a burst with a bright optical counterpart. DATE: 25/09/20 09:01:06 GMT FROM: K.L. Page at U Leicester <klp5@leicester.ac.uk> R. Gupta (NASA GSFC), C. Gronwall (PSU), N. P. M. Kuin (UCL-MSSL) and K. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift Observatory Team: At 08:46:30 UT, the Swift Burst Alert Telescope (BAT) triggered and located GRB 250920B (trigger=1351008). Swift slewed immediately to the burst. The BAT on-board calculated location is RA, Dec 187.337, +47.844 which is RA(J2000) = 12h 29m 21s Dec(J2000) = +47d 50' 39" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve showed multiple peaks, with a complex structure with a duration of about 150 sec. The peak count rate was ~8200 counts/sec (15-350 keV), at ~5 sec after the trigger. The XRT began observing the field at 08:48:01.1 UT, 90.6 seconds after the BAT trigger. XRT found a bright, uncatalogued X-ray source located at RA, Dec 187.3179, 47.8372 which is equivalent to: RA(J2000) = 12h 29m 16.30s Dec(J2000) = +47d 50' 13.9" with an uncertainty of 4.7 arcseconds (radius, 90% containment). This location is 52 arcseconds from the BAT onboard position, within the BAT error circle. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 8.20e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 98 seconds after the BAT trigger. There is a candidate afterglow in the rapidly available 2.7'x2.7' sub-image at RA(J2000) = 12:29:16.07 = 187.31697 DEC(J2000) = +47:50:14.4 = 47.83734 with a 90%-confidence error radius of about 0.74 arc sec. This position is 2.3 arc sec. from the center of the XRT error circle. The estimated magnitude is 13.75 with a 1-sigma error of about 0.14. No correction has been made for extinction. Burst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com). 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/41898. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...