TITLE:   GCN CIRCULAR
NUMBER:  38783
SUBJECT: LIGO/Virgo/KAGRA S241230bd: Upper limits from Swift/BAT-GUANO
DATE:    25/01/02 22:22:30 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 75.9% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S241230bd/files/Bilby.multiorder.fits)) at merger time. A fraction 32.05% 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        |10.80 |7.25  |6.44  |8.08  
|1.024        |5.50  |3.69  |3.28  |4.11  
|4.096        |2.96  |1.99  |1.77  |2.21  
|16.384       |1.84  |1.24  |1.10  |1.38  


The upper limits as function of sky position are plotted here, alongside the GW localization:
https://zenodo.org/records/14589498

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/