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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
We present results from offline searches of Fermi Gamma-ray Burst Monitor
(GBM) data for gamma-ray transients coincident with the compact binary
coalescences observed by the gravitational-wave (GW) detectors Advanced LIGO
and Advanced Virgo during their first and second observing runs. In particular,
we perform follow-up for both confirmed events and low significance candidates
reported in the LIGO/Virgo catalog GWTC-1. We search for temporal coincidences
between these GW signals and GBM triggered gamma-ray bursts (GRBs). We also use
the GBM Untargeted and Targeted subthreshold searches to find coincident
gamma-rays below the on-board triggering threshold. This work implements a
refined statistical approach by incorporating GW astrophysical source
probabilities and GBM visibilities of LIGO/Virgo sky localizations to search
for cumulative signatures of coincident subthreshold gamma-rays. All search
methods recover the short gamma-ray burst GRB 170817A occurring ~1.7 s after
the binary neutron star merger GW170817. We also present results from a new
search seeking GBM counterparts to LIGO single-interferometer triggers. This
search finds a candidate joint event, but given the nature of the GBM signal
and localization, as well as the high joint false alarm rate of $1.1 \times
10^{-6}$ Hz, we do not consider it an astrophysical association. We find no
additional coincidences.