GCN Circular 25834
F. Longo (Univ. and INFN Trieste), E. Bissaldi (Politecnico and INFN Bari), E. Moretti (IFAE, Barcelona), M. Moss (George Washington Univ.), N. Omodei (Stanford Univ.), D. Kocevski (NASA/MSFC) and M. Axelsson (KTH and Stockholm Univ.) report on behalf of the Fermi-LAT Collaboration: We have searched data collected by the Fermi Large Area Telescope (LAT) on Sep 23, 2019, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S190923y (GCN 25814). We define "instantaneous coverage" as the integral over the region of the LIGO probability map that is within the LAT field of view at a given time, and "cumulative coverage" as the integral of the instantaneous coverage over time. Fermi-LAT had an instantaneous coverage of ~40% of the LIGO probability region at the time of the trigger (T0 = 2019-09-23 12:55:59.646 UTC) and reached 100% cumulative coverage at approximately T0 + 8.8 ks. We performed a search for a transient counterpart within the observed region of the 90% contour of the LIGO map in a fixed time window from T0 to T0 + 10 ks. No significant sources were found. We also performed a search which adapted the time interval of the analysis to the exposure of each region of the sky, and no additional excesses were found. Energy flux upper bounds for the fixed time interval between 100 MeV and 1 GeV for this search vary between 1.3e-10 and 1.4e-9 [erg/cm^2/s]. The Fermi-LAT point of contact for this event is Elena Moretti (email@example.com). The Fermi-LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.