TITLE: GCN CIRCULAR NUMBER: 34790 SUBJECT: LIGO/Virgo/KAGRA S230928cb: Upper limits from Swift/BAT-GUANO DATE: 23/10/03 14:08:18 GMT FROM: Samuele Ronchini at PSU sjs8171@psu.edu

Samuele Ronchini (PSU), Aaron Tohuvavohu (U Toronto), Gayathri Raman (PSU), Tyler Parsotan (NASA GSFC), James DeLaunay (U Alabama), Jamie A. Kennea (PSU) report:

Swift/BAT was observing 85.5% of the GW localization probability (bayestar.multiorder.fits) at merger time. A fraction 36.5 % 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).

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), 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 11.3 7.72 6.95 8.48 1.024 5.76 3.93 3.54 4.32 4.096 3.08 2.10 1.89 2.31 16.38 1.89 1.28 1.16 1.41

The upper limits as function of sky position are plotted here, alongside the GW localization: https://doi.org/10.5281/zenodo.8397496 The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively.

The corresponding fits file can be found here: https://doi.org/10.5281/zenodo.8397498

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/

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