{
  "subject": "GRB 250509A: Swift detection of a burst",
  "submitter": "Mike Siegel at PSU/Swift MOC <siegel@swift.psu.edu>",
  "circularId": 40407,
  "createdOn": 1746830954196,
  "bibcode": "2025GCN.40407....1S",
  "eventId": "GRB 250509A",
  "body": "M. H. Siegel (PSU), S. Dichiara (PSU), R. Gupta (NASA GSFC) and\nB. Sbarufatti (INAF-OAB) report on behalf of the Neil Gehrels Swift\nObservatory Team:\n\nAt 22:33:30 UT, the Swift Burst Alert Telescope (BAT) triggered and\nlocated GRB 250509A (trigger=1311764). Swift slewed immediately to the burst.\nThe BAT on-board calculated location is RA, Dec 46.455, -38.853 which is \n   RA(J2000) = 03h 05m 49s\n   Dec(J2000) = -38d 51' 11\"\nwith an uncertainty of 3 arcmin (radius, 90% containment, including \nsystematic uncertainty).  The BAT light curve showed a multi peak\nstructure with a duration of about 60 sec.  The peak count rate\nwas ~2800 counts/sec (15-350 keV), at ~0 sec after the trigger.\n\nThe XRT began observing the field at 22:34:41.7 UT, 71.1 seconds after\nthe BAT trigger. Using promptly downlinked data we find a bright,\nuncatalogued X-ray source located at RA, Dec 46.45032, -38.84173 which\nis equivalent to:\n   RA(J2000)  = 03h 05m 48.08s\n   Dec(J2000) = -38d 50' 30.2\"\nwith an uncertainty of 3.6 arcseconds (radius, 90% containment). This\nlocation is 42 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 (2.03 x\n10^20 cm^-2, Willingale et al. 2013), with an excess column of 9.2\n(+3.98/-3.41) x 10^21 cm^-2 (90% confidence). \n\nThe initial flux in the 2.5 s image was 7.21e-09 erg cm^-2 s^-1 (0.2-10\nkeV).\n\nUVOT took a finding chart exposure of nominal 150.000 seconds with the White\nfilter starting 80 seconds after the BAT trigger. No credible afterglow\ncandidate has been found in the initial data products. Data from the 2.7'x2.7'\nsub-image are not available at this time. The 8'x8' region for the list of\nsources generated on-board covers 100% of the XRT error circle. The list of\nsources is typically complete to about 18 mag. No correction has been made for\nthe expected extinction corresponding to E(B-V) of 0.021.\n\nBurst Advocate for this burst is M. H. Siegel (siegel AT swift.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",
  "submittedHow": "email"
}