GCN Circular 36755
Samuele Ronchini (PSU), Gayathri Raman (PSU), Aaron Tohuvavohu (U Toronto), Jamie A. Kennea (PSU), James DeLaunay (PSU), Tyler Parsotan (NASA GSFC) report:
Swift/BAT was observing >99% of the GW localization probability (Bilby.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], 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 | 20.8 | 13.3 | 11.7 | 14.9 |
| 1.024 | 10.6 | 6.78 | 5.98 | 7.61 |
| 4.096 | 5.74 | 3.66 | 3.23 | 4.11 |
| 16.384 | 3.60 | 2.30 | 2.02 | 2.57 |
The upper limits as function of sky position are plotted here, alongside the GW localization: https://doi.org/10.5281/zenodo.12524835
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/