{
  "editedBy": "Judith Racusin at NASA/GSFC <judith.racusin@nasa.gov> on behalf of K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "circularId": 41403,
  "createdOn": 1755481737339,
  "bibcode": "2025GCN.41403....1C",
  "version": 2,
  "editedOn": 1755523881265,
  "format": "text/plain",
  "submittedHow": "email",
  "subject": "GRB 250818A: Swift detection of a burst",
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "eventId": "GRB 250818A",
  "body": "S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),\nH. A. Krimm (NSF), A. Y. Lien (U Tampa), K. L. Page (U Leicester),\nD. M. Palmer (LANL) and M. H. Siegel (PSU) report on behalf of the\nNeil Gehrels Swift Observatory Team:\n\nAt 01:31:22 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250818A (trigger=1343270).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 213.833, -58.007 which is \n   RA(J2000) = 14h 15m 20s\n   Dec(J2000) = -58d 00' 26\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  Due to a communications gap, only the\nBAT light curve after T+8s is available.  This later lightcurve\nshows a complex structure out to at least T+150s.  The peak count\nrate in the available lightcurve was ~5500 counts/sec (15-350 keV), \nat ~80 sec after the trigger.\n\nThe XRT began observing the field at 01:33:00.6 UT, 97.9 seconds after\nthe BAT trigger. XRT found a bright, uncatalogued X-ray source located\nat RA, Dec 213.8002, -58.0134 which is equivalent to:\n   RA(J2000)  = 14h 15m 12.05s\n   Dec(J2000) = -58d 00' 48.2\"\nwith an uncertainty of 4.9 arcseconds (radius, 90% containment). This\nlocation is 66 arcseconds from the BAT onboard position, within the BAT\nerror circle. \n\nThe initial flux in the 0.1 s image was 1.09e-07 erg cm^-2 s^-1 (0.2-10\nkeV).\n\nUVOT took a finding chart exposure of 250 seconds with the U filter starting\n162 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 none of the\nXRT error circle. The 8'x8' region for the list of sources generated on-board\ncovers 100% of the XRT error circle. Because of the density of catalogued\nstars, further analysis is required to report an upper limit for any afterglow\nin the region. No correction has been made for the large, but uncertain,\nextinction expected.\n\nBurst Advocate for this burst is S. B. Cenko (brad.cenko AT nasa.gov).\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"
}