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GCN Circular 34790

LIGO/Virgo/KAGRA S230928cb: Upper limits from Swift/BAT-GUANO
2023-10-03T14:08:18Z (7 months ago)
Samuele Ronchini at PSU <>
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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:
The solid and dashed lines indicate the 90% and 50% GW contour levels, respectively.

The corresponding fits file can be found here:

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:

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