TITLE: GCN CIRCULAR NUMBER: 25102 SUBJECT: LIGO/Virgo S190718y: No counterpart candidates in Fermi-LAT observations DATE: 19/07/19 08:35:27 GMT FROM: Magnus Axelsson at Stockholm U. M. Axelsson (KTH and Stockholm Univ.), E. Bissaldi (Politecnico and INFN Bari), F.Longo (University and INFN, Trieste) and M. Palatiello (University and INFN, Trieste) report on behalf of the Fermi-LAT Collaboration: We have searched data collected by the Fermi Large Area Telescope (LAT) on July 18, 2019, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S190718y (GCN 25087). 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 a time, and "cumulative coverage" as the integral of the instantaneous coverage over time. Fermi-LAT had instantaneous coverage of ~30% of the LIGO probability region at the time of the trigger (T0 = 2019-07-18 14:35:12.068 UTC), and reached ~98% coverage at T0+4 ks. The remaining area was not observed within 10 ks of the trigger. 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 9.8e-11 and 1.1e-07 [erg/cm^2/s]. The Fermi-LAT point of contact for this event is Michele Palatiello (michele.palatiello@gmail.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.