(R. Hamburg, P. Veres, C. M. Hui, D. Kocevski, A. Goldstein)
The Fermi Gamma-ray Burst Monitor (GBM) Team has developed an offline search for weak gamma-ray bursts which were not already detected in-orbit as triggers. This search is untargeted, searching all of the GBM data without guidance from other observations. While the search is optimized to find short GRBs, the search also finds some long GRBs and other transients. A major goal is to provide additional electromagnetic counterparts to LIGO/Virgo detections. GBM detections can increase confidence in weak LIGO/Virgo signals or improve localizations when fewer than three interferometers are operating. Search results can also be correlated with neutrino detections. The search runs automatically upon receipt of the GBM data. The typically several hour delay in receiving the data dominates the latency for detecting transients. The first public version went online in January 2016. Since July 2017 transients found by the search have been distributed via the GCN (https://gcn.gsfc.nasa.gov/admin/fermi_gbm_subthreshold_announce.txt). Transients found by this search, which did not trigger the on-board GBM software, have been confirmed with other instruments. The current version of the search is estimated to more than double the GBM detection rate of short GRBs.