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GCN Circular 36754

LIGO/Virgo/KAGRA S240621dy: Upper limits from Swift/BAT-GUANO
2024-06-25T02:49:31Z (24 days ago)
Samuele Ronchini at PSU <>
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Samuele Ronchini (PSU), James DeLaunay (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea (PSU), Gayathri Raman (PSU), Tyler Parsotan (NASA GSFC) report:

Swift/BAT was observing >99% of the GW localization probability (bayestar.multiorder.fits) at merger time. A fraction <1% 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

Not found
], 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)softnormalhardGRB170817

The upper limits as function of sky position are plotted here, alongside the GW localization:

Not found

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:

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