{
  "submittedHow": "email",
  "bibcode": "2025GCN.40343....1M",
  "circularId": 40343,
  "submitter": "David Palmer at LANL <palmer@lanl.gov>",
  "body": "M. J. Moss (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),\nJ. A. Kennea (PSU), D. M. Palmer (LANL) and M. H. Siegel (PSU) report\non behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 23:25:57 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250504A (trigger=1310284).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 269.612, -40.349 which is \n   RA(J2000) = 17h 58m 27s\n   Dec(J2000) = -40d 20' 57\"\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 60 sec.  The peak count rate\nwas ~4000 counts/sec (15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 23:27:15.3 UT, 77.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find a fading,\nuncatalogued X-ray source with an enhanced position: RA, Dec 269.61691,\n-40.36642 which is equivalent to:\n   RA(J2000)  = 17h 58m 28.06s\n   Dec(J2000) = -40d 21' 59.1\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 64 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. \n\nA power-law fit to a spectrum formed from promptly downlinked event\ndata gives a column density consistent with the Galactic value of 2.12\nx 10^21 cm^-2 (Willingale et al. 2013).\n\nUVOT took a finding chart exposure of 250 seconds with the U filter starting\n137 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 0.00% of the XRT\nerror circle. No correction has been made for the expected extinction\ncorresponding to E(B-V) of 0.191.\n\nBurst Advocate for this burst is M. J. Moss (mikejmoss3 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",
  "createdOn": 1746402256142,
  "eventId": "GRB 250504A",
  "subject": "GRB 250504A: Swift detection of a burst"
}