TITLE: GCN CIRCULAR NUMBER: 37621 SUBJECT: LIGO/Virgo/KAGRA S240921cw: Upper limits from Swift/BAT-GUANO DATE: 24/09/28 00:18:01 GMT FROM: Samuele Ronchini at PSU <sjs8171@psu.edu> Samuele Ronchini (PSU), Aaron Tohuvavohu (U Toronto), James DeLaunay (PSU), Gayathri Raman (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU) report: Swift/BAT was observing 97% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S240921cw/files/Bilby.multiorder.fi...)) at merger time. A fraction 47% of the GW localization posterior is contained inside the BAT coded FoV. The 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)). Upon 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. Using 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. We 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. In units of 10^-7 erg/s/cm^2: |time_bin (s) |soft |normal| hard | GRB170817 |-|-|-|-|-| |0.256 |9.55 | 9.24 | 8.66 | 9.73 |1.024 |9.24 | 8.94 | 8.38 | 9.41 |4.096 |7.14 | 6.91 | 6.47 | 7.27 |16.384 |2.49 | 2.41 | 2.25 | 2.53 The upper limits as function of sky position are plotted here, alongside the GW localization: [https://doi.org/10.5281/zenodo.13851953]() The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively. The corresponding fits file is also included. GUANO 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. A live reporting of Swift/BAT event data recovered by GUANO can be found at: [https://www.swift.psu.edu/guano/]() View this GCN Circular online at https://gcn.nasa.gov/circulars/37621. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...