{
  "body": "R. Gupta (NASA GSFC), C. Gronwall (PSU), N. P. M. Kuin (UCL-MSSL) and\nK. L. Page (U Leicester) report on behalf of the Neil Gehrels Swift\nObservatory Team:\n\nAt 08:46:30 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250920B (trigger=1351008).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 187.337, +47.844 which is \n   RA(J2000) = 12h 29m 21s\n   Dec(J2000) = +47d 50' 39\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed multiple peaks, with a\ncomplex structure with a duration of about 150 sec.  The peak count rate\nwas ~8200 counts/sec (15-350 keV), at ~5 sec after the trigger.\n\nThe XRT began observing the field at 08:48:01.1 UT, 90.6 seconds after\nthe BAT trigger. XRT found a bright, uncatalogued X-ray source located\nat RA, Dec 187.3179, 47.8372 which is equivalent to:\n   RA(J2000)  = 12h 29m 16.30s\n   Dec(J2000) = +47d 50' 13.9\"\nwith an uncertainty of 4.7 arcseconds (radius, 90% containment). This\nlocation is 52 arcseconds from the BAT onboard position, within the BAT\nerror circle. No event data are yet available to determine the column\ndensity using X-ray spectroscopy.\n\nThe initial flux in the 2.5 s image was 8.20e-09 erg cm^-2 s^-1 (0.2-10\nkeV).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 98 seconds after the BAT trigger. There is a candidate afterglow in\nthe rapidly available 2.7'x2.7' sub-image at\n  RA(J2000)  =\t12:29:16.07 = 187.31697\n  DEC(J2000) = +47:50:14.4  =  47.83734\nwith a 90%-confidence error radius of about 0.74 arc sec. This position is 2.3\narc sec. from the center of the XRT error circle. The estimated magnitude is\n13.75 with a 1-sigma error of about  0.14. No correction has been made for\nextinction.\n\nBurst Advocate for this burst is R. Gupta (rahulbhu.c157 AT gmail.com).\nPlease contact the BA by email if you require additional information\nregarding Swift followup of this burst. In extremely urgent cases, after\ntrying the Burst Advocate, you can contact the Swift PI by phone (see\nSwift TOO web site for information: http://www.swift.psu.edu/)\n\n",
  "bibcode": "2025GCN.41898....1G",
  "circularId": 41898,
  "createdOn": 1758358866844,
  "eventId": "GRB 250920B",
  "submittedHow": "email",
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "subject": "GRB 250920B: Swift detection of a burst with a bright optical counterpart."
}