{
  "createdOn": 1734483502774,
  "circularId": 38601,
  "bibcode": "2024GCN.38601....1W",
  "subject": "GRB 241218A: Swift detection of a burst",
  "submittedHow": "email",
  "eventId": "GRB 241218A",
  "submitter": "Boris Sbarufatti at INAF-OAB <boris.sbarufatti@inaf.it>",
  "body": "M. A. Williams (PSU), R. Gupta (NASA GSFC), N. P. M. Kuin (UCL-MSSL),\nB. Sbarufatti (INAF-OAB) and A. Tohuvavohu (Caltech) report on behalf\nof the Neil Gehrels Swift Observatory Team:\n\nAt 00:08:51 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 241218A (trigger=1275005).  Swift slewed immediately to the burst.\nThe BAT on-board calculated location is \nRA, Dec 185.682, +29.991 which is \n   RA(J2000) = 12h 22m 44s\n   Dec(J2000) = +29d 59' 27\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a complex\nstructure with a duration of about 25 sec.  The peak count rate\nwas ~896 counts/sec (15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 00:11:15.6 UT, 144.5 seconds after\nthe BAT trigger. The position determined from promptly downlinked data\ndiffers significantly from the on-board position, suggesting that the\nXRT may have centroided on a cosmic ray; the initial XRT position\nnotice should be treated with caution. Using promptly downlinked data\nwe find a fading, uncatalogued X-ray source with an enhanced position:\nRA, Dec 185.68146, 29.97303 which is equivalent to:\n   RA(J2000)  = 12h 22m 43.55s\n   Dec(J2000) = +29d 58' 22.9\"\nwith an uncertainty of 1.9 arcseconds (radius, 90% containment). This\nlocation is 64 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 consistent with the Galactic value of 2.23\nx 10^20 cm^-2 (Willingale et al. 2013).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 151 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\n0.023.\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"
}