TITLE:   GCN CIRCULAR
NUMBER:  37884
SUBJECT: LIGO/Virgo/KAGRA S241009l: Upper limits from Swift/BAT-GUANO
DATE:    24/10/25 19:35:36 GMT
FROM:    Maia Williams at PSU <mjw6837@psu.edu>

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

Swift/BAT was observing >99% of the GW localization probability ([Bilby.multiorder.fits](https://gracedb.ligo.org/api/superevents/S241009l/files/Bilby.multiorder.fits)) at merger time. A fraction 90% 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        |2.50| 2.14  |  1.92 |2.34
|1.024        |1.28| 1.09  |  0.98 |1.19
|4.096        |0.69| 0.59  |  0.53 |0.64
|16.384       |0.43| 0.37  |  0.33 |0.40

The upper limits as function of sky position are plotted here, alongside the GW localization:
[https://doi.org/10.5281/zenodo.13992839]()

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/]()