{
  "bibcode": "2025GCN.41303....1P",
  "body": "T. M. Parsotan (GSFC), R. Brivio (INAF-OAB), J. J. DeLaunay (PSU),\nS. Dichiara (PSU), R. Gupta (NASA GSFC) and D. M. Palmer (LANL) report\non behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 16:53:19 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250809A (trigger=1340954).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 233.015, -53.289 which is \n   RA(J2000) = 15h 32m 04s\n   Dec(J2000) = -53d 17' 18\"\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 40 sec.  The peak count rate\nwas ~1000 counts/sec (15-350 keV), at ~11 sec after the trigger.\n\nThe XRT began observing the field at 16:55:25.0 UT, 125.5 seconds after\nthe BAT trigger. Using promptly downlinked data we find a bright,\nuncatalogued X-ray source located at RA, Dec 233.00282, -53.29640 which\nis equivalent to:\n   RA(J2000)  = 15h 32m 00.68s\n   Dec(J2000) = -53d 17' 47.0\"\nwith an uncertainty of 4.3 arcseconds (radius, 90% containment). This\nlocation is 37 arcseconds from the BAT onboard position, within the BAT\nerror circle. This position may be improved as more data are received;\nthe latest position is available at https://www.swift.ac.uk/sper.  We\ncannot determine whether the source is fading at the present time. \n\nA power-law fit to a spectrum formed from promptly downlinked event\ndata gives a column density consistent with the Galactic value of 8.04\nx 10^21 cm^-2 (Willingale et al. 2013). \n\nThe initial flux in the 2.5 s image was 5.16e-10 erg cm^-2 s^-1 (0.2-10\nkeV).\n\nUVOT took a finding chart exposure of 250 seconds with the U filter starting\n128 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. Because of the density of catalogued stars, further analysis\nis required to report an upper limit for any afterglow in the sub-image. The\n8'x8' region for the list of sources generated on-board covers 100% of the XRT\nerror circle. Because of the density of catalogued stars, further analysis is\nrequired to report an upper limit for any afterglow in the region. No\ncorrection has been made for the large, but uncertain, extinction expected.\n\nBurst Advocate for this burst is T. M. Parsotan (tyler.parsotan 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",
  "circularId": 41303,
  "eventId": "GRB 250809A",
  "subject": "GRB 250809A: Swift detection of a gamma ray burst",
  "submitter": "David Palmer at LANL <palmer@lanl.gov>",
  "createdOn": 1754759351264,
  "submittedHow": "email"
}