{
  "subject": "GRB 250331B: Swift detection of a burst",
  "eventId": "GRB 250331B",
  "submittedHow": "email",
  "bibcode": "2025GCN.39958....1C",
  "createdOn": 1743396451008,
  "circularId": 39958,
  "submitter": "David Palmer at LANL <palmer@lanl.gov>",
  "body": "S. B. Cenko (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),\nK. L. Page (U Leicester), D. M. Palmer (LANL) and M. H. Siegel (PSU)\nreport on behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 04:19:43 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250331B (trigger=1299965).  Swift slewed to the burst\nafter a brief delay to clear an observing constraint.\nThe BAT on-board calculated location is \nRA, Dec 71.381, +43.805 which is \n   RA(J2000) = 04h 45m 31s\n   Dec(J2000) = +43d 48' 17\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a single-peak\nstructure with a duration of about 8 sec.  The peak count rate\nwas ~1400 counts/sec (15-350 keV), at ~2 sec after the trigger.\n\nThe XRT began observing the field at 04:24:57.3 UT, 313.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find an uncatalogued\nX-ray source with an enhanced position: RA, Dec 71.37364, 43.81080\nwhich is equivalent to:\n   RA(J2000)  = 04h 45m 29.67s\n   Dec(J2000) = +43d 48' 38.9\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 28 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 does not constrain the column density.\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 317 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 100% of\nthe XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag.\nThe 8'x8' region for the list of sources generated on-board covers 100% of the\nXRT error circle. The list of sources is typically complete to about 18 mag. No\ncorrection has been made for the large, but uncertain, 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"
}