{
  "submittedHow": "email",
  "createdOn": 1746036427508,
  "submitter": "Tyler Parsotan  at NASA GSFC <tyler.parsotan@nasa.gov>",
  "eventId": "GRB 250430A",
  "subject": "GRB 250430A: Swift detection of a burst with optical counterpart",
  "body": "T. M. Parsotan (GSFC), C. Gronwall (PSU), R. Gupta (NASA GSFC),\nN. J. Klingler (GSFC/UMBC/CRESSTII), M. J. Moss (GSFC),\nK. L. Page (U Leicester), M. H. Siegel (PSU) and M. A. Williams (PSU)\nreport on behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 17:31:21 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250430A (trigger=1308754). Swift slewed immediately to the burst.\nThe BAT on-board calculated location is RA, Dec 233.428, -18.083 which is \n   RA(J2000) = 15h 33m 43s\n   Dec(J2000) = -18d 04' 59\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a multi peak\nstructure with a duration of about 20 sec.  The peak count rate\nwas ~3000 counts/sec (15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 17:33:37.5 UT, 157.3 seconds after\nthe BAT trigger. Using promptly downlinked data we find an uncatalogued\nX-ray source located at RA, Dec 233.38611, -18.11784 which is\nequivalent to:\n   RA(J2000)  = 15h 33m 32.67s\n   Dec(J2000) = -18d 07' 04.2\"\nwith an uncertainty of 3.6 arcseconds (radius, 90% containment). This\nlocation is 190 arcseconds from the BAT onboard position, outside 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 in excess of the Galactic value (9.57 x\n10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.5\n(+2.62/-2.27) x 10^21 cm^-2 (90% confidence).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 161 seconds after the BAT trigger. There is a candidate afterglow in\nthe list of sources generated on-board at\nRA(J2000) = 15:33:32.85 = 233.38688\nDEC(J2000) = -18:07:06.2 = -18.11840\nwith a 90%-confidence error radius of about 1.10 arc sec. This position is 3.65\narc sec. from the center of the XRT error circle. The estimated magnitude is\n19.44 with a 1-sigma error of about 0.35. No correction has been made for the\nexpected extinction corresponding to E(B-V) of 0.092.\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": 40292,
  "bibcode": "2025GCN.40292....1P"
}