{
  "format": "text/markdown",
  "body": "Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:\n\nSwift/BAT was observing 100.0% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S250331o/files/Bilby.multiorder.fits)) at merger time. A fraction 96.02% of the GW localization posterior is contained inside the BAT coded FoV.\n\nThe LVK 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](https://iopscience.iop.org/article/10.3847/1538-4357/aba94f)).\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\nUsing the NITRATES analysis ([DeLaunay + Tohuvavohu 2022, ApJ, 941, 169](https://iopscience.iop.org/article/10.3847/1538-4357/ac9d38)), we searched for emission on 8 timescales from 0.128s to 16.384s in the interval [-20,+20] seconds around the merger time. We find no evidence for a signal, and derive the following upper limits.\n\nWe quote the 5-sigma flux upper limits in the 15-350 keV band, weighted over the GW localization, for four spectral templates (soft, normal, and hard GRB-like templates described in [arXiv:1612.02395], and spectral shape from GRB170817A [arXiv:1710.05446]) and for four time bins.\nIn units of 10^-7 erg/s/cm^2:\n\n|time_bin (s) |soft  |normal|hard  |GRB170817\n|-|-|-|-|-|\n|0.256        |3.32  |3.49  |3.33  |3.61  \n|1.024        |1.69  |1.78  |1.70  |1.84  \n|4.096        |0.91  |0.95  |0.91  |0.99  \n|16.384       |0.55  |0.58  |0.56  |0.60  \n\n\nThe upper limits as function of sky position are plotted here, alongside the GW localization:\nhttps://zenodo.org/records/15169889\n\nThe solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included.\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: https://www.swift.psu.edu/guano/",
  "subject": "LIGO/Virgo/KAGRA S250331o: Upper limits from Swift/BAT-GUANO",
  "eventId": "LIGO/Virgo/KAGRA S250331o",
  "createdOn": 1744052697967,
  "submittedHow": "web",
  "bibcode": "2025GCN.40109....1W",
  "circularId": 40109,
  "submitter": "Maia Williams at PSU <mjw6837@psu.edu>"
}