{
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "circularId": 37466,
  "bibcode": "2024GCN.37466....1E",
  "body": "P. A. Evans (U Leicester), R. Gupta (NASA GSFC), J. A. Kennea (PSU),\nK. L. Page (U Leicester) and D. M. Palmer (LANL) report on behalf of\nthe Neil Gehrels Swift Observatory Team:\n\nAt 01:46:39 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 240912A (trigger=1253910).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 124.633, +33.995 which is \n   RA(J2000) = 08h 18m 32s\n   Dec(J2000) = +33d 59' 42\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a complex\nstructure with a duration of at least 150 sec, extending into\nthe observation times of the XRT and UVOT.  The peak count rate\nwas ~17000 counts/sec (15-350 keV), at ~69 sec after the trigger.\n\nThe XRT began observing the field at 01:48:25.2 UT, 106.0 seconds after\nthe BAT trigger. Using promptly downlinked data we find a bright,\nfading, uncatalogued X-ray source located at RA, Dec 124.63711,\n33.99865 which is equivalent to:\n   RA(J2000)  = 08h 18m 32.91s\n   Dec(J2000) = +33d 59' 55.1\"\nwith an uncertainty of 3.5 arcseconds (radius, 90% containment). This\nlocation is 17 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 (5.53 x\n10^20 cm^-2, Willingale et al. 2013), with an excess column of 4.2\n(+2.90/-2.49) x 10^21 cm^-2 (90% confidence). \n\nThe initial flux in the 0.1 s image was 8.64e-08 erg cm^-2 s^-1 (0.2-10\nkeV).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 115 seconds after the BAT trigger. There is a candidate afterglow in\nthe rapidly available 2.7'x2.7' sub-image at\n  RA(J2000)  =  08:18:32.40 = 124.63502\n  DEC(J2000) = +33:59:56.3  =  33.99896\nwith a 90%-confidence error radius of about 0.75 arc sec. This position is 8.0\narc sec. from the center of the XRT error circle. The estimated magnitude is\n17.40 with a 1-sigma error of about  0.14. No correction has been made for the\nexpected extinction corresponding to E(B-V) of 0.054.\n\nBurst Advocate for this burst is P. A. Evans (pae9 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",
  "createdOn": 1726107186988,
  "submittedHow": "email",
  "eventId": "GRB 240912A",
  "subject": "GRB 240912A: Swift detection of a burst with an optical counterpart"
}