{
  "eventId": "GRB 240421B",
  "circularId": 36208,
  "subject": "GRB 240421B: Swift detection of a burst",
  "bibcode": "2024GCN.36208....1W",
  "body": "\nM. A. Williams (PSU), J. J. DeLaunay (PSU), S. Dichiara (PSU),\nJ.D. Gropp (PSU) and T. Sakamoto (AGU) report on behalf of the Neil\nGehrels Swift Observatory Team:\n\nAt 09:42:54 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 240421B (trigger=1223470).  Swift slewed immediately to the burst. \nThe BAT on-board calculated location is \nRA, Dec 299.686, -14.879 which is \n   RA(J2000) = 19h 58m 45s\n   Dec(J2000) = -14d 52' 42\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a multi-peaked\nstructure with a duration of about 20 sec.  The peak count rate\nwas ~9000 counts/sec (15-350 keV), at ~0 sec after the trigger. \n\nThe XRT began observing the field at 09:45:00.4 UT, 126.3 seconds after\nthe BAT trigger. Using promptly downlinked data we find an uncatalogued\nX-ray source with an enhanced position: RA, Dec 299.70011, -14.86012\nwhich is equivalent to:\n   RA(J2000)  = 19h 58m 48.03s\n   Dec(J2000) = -14d 51' 36.4\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 83 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 (1.24 x\n10^21 cm^-2, Willingale et al. 2013), with an excess column of 3.8\n(+2.62/-2.25) x 10^21 cm^-2 (90% confidence). \n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 129 seconds after the BAT trigger. No credible afterglow candidate has\nbeen found in the initial data products. The 2.7'x2.7' sub-image covers 100% of\nthe XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. \nThe 8'x8' region for the list of sources generated on-board covers 100% of the\nXRT error circle. The list of sources is typically complete to about 18 mag. No\ncorrection has been made for the expected extinction corresponding to E(B-V) of\n0.183. \n\nBurst Advocate for this burst is M. A. Williams (mjw6837 AT psu.edu). \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": 1713693738097,
  "submitter": "Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu>",
  "submittedHow": "email"
}