{
  "body": "S. Lanava (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC),\nM. J. Moss (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL) and\nB. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift\nObservatory Team:\n\nAt 16:46:36 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 251103B (trigger=1409824).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 105.864, -24.843 which is \n   RA(J2000) = 07h 03m 27s\n   Dec(J2000) = -24d 50' 35\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a complex\nstructure with a duration of about 200 sec.  The peak count rate\nwas ~4000 counts/sec (15-350 keV), at ~2 sec after the trigger.\n\nThe XRT began observing the field at 16:47:49.2 UT, 72.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find a bright,\nuncatalogued X-ray source located at RA, Dec 105.87352, -24.87083 which\nis equivalent to:\n   RA(J2000)  = 07h 03m 29.64s\n   Dec(J2000) = -24d 52' 15.0\"\nwith an uncertainty of 3.6 arcseconds (radius, 90% containment). This\nlocation is 104 arcseconds from the BAT onboard position, within the\nBAT error circle. This position may be improved as more data are\nreceived; the latest position is available at\nhttps://www.swift.ac.uk/sper. \n\nA power-law fit to a spectrum formed from promptly downlinked event\ndata gives a column density in excess of the Galactic value (3.33 x\n10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.5\n(+2.76/-2.40) x 10^21 cm^-2 (90% confidence). \n\nThe initial flux in the 2.5 s image was 2.78e-09 erg cm^-2 s^-1 (0.2-10\nkeV).\n\nUVOT took a finding chart exposure of 250 seconds with the U filter starting\n139 seconds after the BAT trigger. No credible afterglow candidate has been\nfound in the initial data products. The 2.7'x2.7' sub-image covers 100% of the\nXRT error circle. The typical 3-sigma upper limit has been about 19.2 mag. The\n8'x8' region for the list of sources generated on-board covers 100% of the XRT\nerror circle. The list of sources is typically complete to about 18.0 mag. No\ncorrection has been made for the expected extinction corresponding to E(B-V) of\n0.255.\n\nBurst Advocate for this burst is S. Lanava (sml7284 AT psu.edu).\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",
  "submittedHow": "email",
  "createdOn": 1762189411202,
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "eventId": "GRB 251103B",
  "bibcode": "2025GCN.42556....1L",
  "circularId": 42556,
  "subject": "GRB 251103B: Swift detection of a burst"
}