{
  "circularId": 39004,
  "submittedHow": "web",
  "createdOn": 1737423662415,
  "body": "Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (Caltech), Gayathri Raman (PSU), Jamie A. Kennea (PSU), Tyler Parsotan (NASA GSFC) report: \n\nSwift/BAT did not localize GRB 250118A onboard (T0: 2025-11-18T18:31:11.97 UTC, Fermi GCN 38976) \n\nThe Fermi notice, distributed in near real-time, triggered the Swift Mission Operations Center operated Gamma-ray Urgent Archiver for Novel Opportunities (GUANO; Tohuvavohu et al. 2020, ApJ, 900, 1). \n\nUpon trigger by this notice, GUANO sent a command to the Swift Burst Alert Telescope (BAT) to save 200 seconds of BAT event-mode data from [-50,+150] seconds around the time of the burst. All the requested event mode data was delivered to the ground. \n\nThe BAT likelihood search, NITRATES (DeLaunay + Tohuvavohu 2022, ApJ, 941, 169), performed on the temporal window [T0-20 s, T0+20 s], detects the burst with a sqrt(TS) of 13.4 in a 0.128 s analysis time bin, starting at T0 - 0.064 s. \n\nUsing the NITRATES analysis, parameter estimation was performed to obtain the localization of this burst in the form of a HEALPIX Multi-Order Coverage (MOC) skymap. This localization accounts for both statistical and systematic errors. More details in the creation and calibration of these maps will soon be published (DeLaunay et al. 2025. in prep)\n\nThe 90% credible area is 7,422 deg2 and the 50% credible area is 1,652 deg2.\nThe integrated probability inside the coded field of view is <1%. \n\nThe NITRATES skymap is consistent with the Fermi localization reported in GCN 38976. The joint NITRATES+Fermi 90% credible area is 565 deg2 and the 50% credible area is 111 deg2 \n\nA plot of the probability skymap can be viewed here:\n\n[skymap_plot](https://guano.swift.psu.edu/trigger_report?id=758917906/#:~:text=Probability%20Skymap)\n\nThe probability skymap file can be downloaded from the link here\n\n[skymap_fits_file](https://guano.swift.psu.edu/files/758917906/0_n_PROBMAP)\n\nInstructions on how to read and manipulate this map can be found here:\n\nhttps://guano.swift.psu.edu/documentation\n\nMore details about this burst can be found on the trigger report page here:\n\nhttps://guano.swift.psu.edu/trigger_report?id=758917906\n\nGUANO is a fully autonomous, extremely low latency, spacecraft commanding pipeline designed for targeted recovery of BAT event mode data around the times of compelling astrophysical events to enable more sensitive GRB searches. \n\nA live reporting of Swift/BAT event data recovered by GUANO can be found at: \nhttps://www.swift.psu.edu/guano/",
  "bibcode": "2025GCN.39004....1R",
  "eventId": "GRB 250118A",
  "format": "text/markdown",
  "subject": "GRB 250118A: Swift/BAT-GUANO localization skymap of a short burst",
  "submitter": "Samuele Ronchini at PSU <sjs8171@psu.edu>"
}