{
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "subject": "GRB 230427A: Swift detection of a burst with an optical counterpart",
  "eventId": "GRB 230427A",
  "circularId": 33687,
  "createdOn": 1682592715445,
  "body": "\nE. Troja (UToV), P. A. Evans (U Leicester),\nR. A. J. Eyles-Ferris (U Leicester), C. Gronwall (PSU),\nJ.D. Gropp (PSU), J. A. Kennea (PSU) and K. L. Page (U Leicester)\nreport on behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 10:31:52 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 230427A (trigger=1165948).  Swift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 153.324, +48.180 which is \n   RA(J2000) = 10h 13m 18s\n   Dec(J2000) = +48d 10' 48\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  No BAT light-curve is available at this time. \n\nThe XRT began observing the field at 10:33:25.9 UT, 94.0 seconds after\nthe BAT trigger. Using promptly downlinked data we find a fading,\nuncatalogued X-ray source with an enhanced position: RA, Dec 153.31199,\n48.20404 which is equivalent to:\n   RA(J2000)  = 10h 13m 14.88s\n   Dec(J2000) = +48d 12' 14.6\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 91 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 (8.35 x\n10^19 cm^-2, Willingale et al. 2013), with an excess column of 3.5\n(+2.81/-2.63) x 10^21 cm^-2 (90% confidence). \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 98 seconds after the BAT trigger. There is a candidate afterglow in\nthe rapidly available 2.7'x2.7' sub-image at\n  RA(J2000)  =\t10:13:15.05 = 153.31270\n  DEC(J2000) = +48:12:14.4  =  48.20400\nwith a 90%-confidence error radius of about 0.74 arc sec. This position is 1.4\narc sec. from the center of the XRT error circle. The estimated magnitude is\n20.31 with a 1-sigma error of about  0.20. No correction has been made for the\nexpected extinction corresponding to E(B-V) of 0.007. \n\nBurst Advocate for this burst is E. Troja (nora.gsfc AT gmail.com). \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",
  "bibcode": "2023GCN.33687....1T"
}