{
  "bibcode": "2025GCN.40491....1E",
  "subject": "GRB 250520A: Swift detection of a burst",
  "body": "R. A. J. Eyles-Ferris (U Leicester), N. P. M. Kuin (UCL-MSSL),\nA. Y. Lien (U Tampa), M. J. Moss (GSFC) and K. L. Page (U Leicester)\nreport on behalf of the Neil Gehrels Swift Observatory Team:\n\nAt 02:42:16.84 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250520A (trigger=1315630). Swift slewed immediately to the burst.\nThe BAT on-board calculated location is RA, Dec 282.268, -11.847 which is \n   RA(J2000) = 18h 49m 04s\n   Dec(J2000) = -11d 50' 49\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a short burst\nwith a duration of about 1 sec.  The peak count rate\nwas ~10,000 counts/sec (15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 02:44:40.8 UT, 144.1 seconds after\nthe BAT trigger. Using promptly downlinked data we find a fading,\nuncatalogued X-ray source located at RA, Dec 282.28588, -11.86811 which\nis equivalent to:\n   RA(J2000)  = 18h 49m 08.61s\n   Dec(J2000) = -11d 52' 05.2\"\nwith an uncertainty of 3.6 arcseconds (radius, 90% containment). This\nlocation is 98 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 (3.68 x\n10^21 cm^-2, Willingale et al. 2013), with an excess column of 8\n(+6.16/-4.83) x 10^21 cm^-2 (90% confidence).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 148 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. Because of the density of catalogued stars, further\nanalysis is required to report an upper limit for any afterglow in the\nsub-image. The 8'x8' region for the list of sources generated on-board covers\n100% of the XRT error circle. Because of the density of catalogued stars,\nfurther analysis is required to report an upper limit for any afterglow in the\nregion. No correction has been made for the large, but uncertain, extinction\nexpected.\n\nBurst Advocate for this burst is R. A. J. Eyles-Ferris (raje1 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",
  "circularId": 40491,
  "submitter": "K.L. Page at U Leicester <klp5@leicester.ac.uk>",
  "eventId": "GRB 250520A",
  "submittedHow": "email",
  "createdOn": 1747709998264
}