{
  "eventId": "GRB 250331C",
  "circularId": 39959,
  "subject": "GRB 250331C: Swift detection of a burst",
  "body": "S. B. Cenko (GSFC), C. Gronwall (PSU), 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 04:37:40 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250331C (trigger=1299967).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 80.993, +33.029 which is \n   RA(J2000) = 05h 23m 58s\n   Dec(J2000) = +33d 01' 43\"\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 25 sec.  The peak count rate\nwas ~800 counts/sec (15-350 keV), at ~1 sec after the trigger.\n\nThe XRT began observing the field at 04:39:51.5 UT, 130.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find a bright,\nuncatalogued X-ray source located at RA, Dec 80.95740, 33.06310 which\nis equivalent to:\n   RA(J2000)  = 05h 23m 49.78s\n   Dec(J2000) = +33d 03' 47.2\"\nwith an uncertainty of 3.7 arcseconds (radius, 90% containment). This\nlocation is 163 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 consistent with the Galactic value of 6.49\nx 10^21 cm^-2 (Willingale et al. 2013). \n\nThe initial flux in the 2.5 s image was 1.04e-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 139 seconds after the BAT trigger. No credible afterglow candidate has\nbeen found in the initial data products. The 2.7'x2.7' sub-image covers none of\nthe XRT error circle. The 8'x8' region for the list of sources generated\non-board covers 100% of the XRT error circle. The list of sources is typically\ncomplete to about 18 mag. No correction has been made for the large, but\nuncertain, extinction 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",
  "submittedHow": "email",
  "bibcode": "2025GCN.39959....1C",
  "submitter": "David Palmer at LANL <palmer@lanl.gov>",
  "createdOn": 1743396940944
}