{
  "submittedHow": "email",
  "createdOn": 1740109995125,
  "subject": "GRB 250221A: Swift detection of a burst with an optical counterpart",
  "submitter": "David Palmer at LANL <palmer@lanl.gov>",
  "bibcode": "2025GCN.39396....1C",
  "circularId": 39396,
  "body": "R. Caputo (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),\nK. L. Page (U Leicester), D. M. Palmer (LANL), T. M. Parsotan (GSFC)\nand M. H. Siegel (PSU) report on behalf of the Neil Gehrels Swift\nObservatory Team:\n\nAt 03:34:37 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250221A (trigger=1290305).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 59.477, -15.139 which is \n   RA(J2000) = 03h 57m 54s\n   Dec(J2000) = -15d 08' 18\"\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 5 sec.  The peak count rate\nwas ~2200 counts/sec (15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 03:36:39.9 UT, 122.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find an uncatalogued\nX-ray source with an enhanced position: RA, Dec 59.46251, -15.13379\nwhich is equivalent to:\n   RA(J2000)  = 03h 57m 51.00s\n   Dec(J2000) = -15d 08' 01.6\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 53 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 in excess of the Galactic value (3.73 x\n10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.1\n(+2.63/-2.31) x 10^21 cm^-2 (90% confidence).\n\nUVOT took a finding chart exposure of 250 seconds with the U filter starting\n340 seconds after the BAT trigger. There is a candidate afterglow in the\nrapidly available 2.7'x2.7' sub-image at\n  RA(J2000)  =\t03:57:51.06 =  59.46274\n  DEC(J2000) = -15:07:59.2  = -15.13311\nwith a 90%-confidence error radius of about 0.76 arc sec. This position is 2.4\narc sec. from the center of the XRT error circle. The estimated magnitude is\n17.82 with a 1-sigma error of about  0.15. No correction has been made for the\nexpected extinction corresponding to E(B-V) of 0.046.\n\nBurst Advocate for this burst is R. Caputo (regina.caputo 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",
  "eventId": "GRB 250221A"
}