{
  "submitter": "Boris Sbarufatti at INAF-OAB <boris.sbarufatti@inaf.it>",
  "circularId": 40585,
  "bibcode": "2025GCN.40585....1C",
  "eventId": "GRB 250530C",
  "createdOn": 1748632984688,
  "submittedHow": "email",
  "subject": "GRB 250530C: Swift detection of a burst",
  "body": "R. Caputo (GSFC), R. Caputo (NASA/GSFC), R. Gupta (NASA GSFC),\nJ. A. Kennea (PSU), M. J. Moss (GSFC), K. L. Page (U Leicester),\nD. M. Palmer (LANL), T. M. Parsotan (GSFC), C. Salvaggio (INAF-OAB),\nT. Sbarrato (INAF-OAB) and B. Sbarufatti (INAF-OAB) report on behalf\nof the Neil Gehrels Swift Observatory Team:\n\nAt 18:58:42 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250530C (trigger=1319125). Swift slewed immediately to the burst.\nThe BAT on-board calculated location is RA, Dec 55.143, -22.459 which is \n   RA(J2000) = 3h 40m 34s\n   Dec(J2000) = -22d 27' 32\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  Due to a telemetry gap, the immediately-available\nBAT light curve does not start until T+8s.  This shows possible activity out\nto T+40 s.  The count rate was ~200 counts/sec (15-350 keV), at\nthe start of available light curve. A more complete description of\nthe emission, including the trigger time, will require the full\ndownlinked data from the next ground pass.\n\nThe XRT began observing the field at 19:00:29.8 UT, 106.9 seconds after\nthe BAT trigger. Using promptly downlinked data we find a fading,\nuncatalogued X-ray source with an enhanced position: RA, Dec 55.17029,\n-22.44825 which is equivalent to:\n   RA(J2000)  = 03h 40m 40.87s\n   Dec(J2000) = -22d 26' 53.7\"\nwith an uncertainty of 1.9 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 (1.70 x\n10^20 cm^-2, Willingale et al. 2013), with an excess column of 3.6\n(+2.00/-1.76) x 10^21 cm^-2 (90% confidence).\n\nUVOT took a finding chart exposure of 150 seconds with the White filter\nstarting 111 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\n0.025.\n\nBurst Advocate for this burst is R. Caputo (regina.caputo AT nasa.gov).\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"
}