TITLE:   GCN CIRCULAR
NUMBER:  38272
SUBJECT: LIGO/Virgo/KAGRA S241116cq: Upper limits from Swift/BAT-GUANO
DATE:    24/11/18 21:34:24 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

Maia Williams (PSU), Samuele Ronchini (PSU), Gayathri Raman (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC), Jamie A. Kennea (PSU), Aaron Tohuvavohu (Caltech) report:

Swift/BAT was observing 97.22% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S241116cq/files/Bilby.offline0.multiorder.fits)) at merger time. A fraction 78.21% 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 |3.23 |2.72 |2.49 |2.94
|1.024 |1.65 |1.39 |1.27 |1.50
|4.096 |0.89 |0.75 |0.68 |0.80
|16.384 |0.55 |0.46 |0.42 |0.50


The upper limits as function of sky position are plotted here, alongside the GW localization:
[https://doi.org/10.5281/zenodo.14182586]()

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/]()