{
  "createdOn": 1750195467935,
  "submitter": "David Palmer at LANL <palmer@lanl.gov>",
  "bibcode": "2025GCN.40760....1P",
  "circularId": 40760,
  "eventId": "GRB 250617B",
  "submittedHow": "email",
  "subject": "GRB 250617B: Swift detection of a burst with an optical counterpart",
  "body": "K. L. Page (U Leicester), R. Gupta (NASA GSFC), A. Y. Lien (U Tampa),\nM. J. Moss (GSFC), D. M. Palmer (LANL), T. Sakamoto (AGU),\nM. H. Siegel (PSU) and M. A. Williams (PSU) report on behalf of the\nNeil Gehrels Swift Observatory Team:\n\nAt 21:01:50.11 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250617B (trigger=1325580). Swift did not slew immediately to the burst.\nThe BAT on-board calculated location is RA, Dec 332.902, +32.727 which is \n   RA(J2000) = 22h 11m 37s\n   Dec(J2000) = +32d 43' 38\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The currently available BAT light curve \nshowed a complex structure with a duration of at least 10 sec. \nHowever, the light curve data from ~T+8 s to ~T+100 s is unavailable \nin the immediate data downlink.  The peak count rate was ~900 counts/sec \n(15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 21:15:03.4 UT, 793.4 seconds after\nthe BAT trigger. Using promptly downlinked data we find an uncatalogued\nX-ray source with an enhanced position: RA, Dec 332.89735, 32.73270\nwhich is equivalent to:\n   RA(J2000)  = 22h 11m 35.36s\n   Dec(J2000) = +32d 43' 57.7\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 24 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 gives a column density in excess of the Galactic value (9.66 x\n10^20 cm^-2, Willingale et al. 2013), with an excess column of 5.7\n(+4.15/-3.46) x 10^21 cm^-2 (90% confidence).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 796 seconds after the BAT trigger. There is a candidate afterglow in\nthe rapidly available 2.7'x2.7' sub-image at\n  RA(J2000)  =\t22:11:35.25 = 332.89686\n  DEC(J2000) = +32:43:56.9  =  32.73247\nwith a 90%-confidence error radius of about 0.62 arc sec. This position is 1.9\narc sec. from the center of the XRT error circle. The estimated magnitude is\n18.17 with a 1-sigma error of about  0.15. No correction has been made for the\nexpected extinction corresponding to E(B-V) of 0.087.\n\nBurst Advocate for this burst is K. L. Page (klp5 AT leicester.ac.uk).\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"
}