TITLE: GCN CIRCULAR NUMBER: 25763 SUBJECT: LIGO/Virgo S190915ak: No counterpart candidates in Fermi-LAT observations DATE: 19/09/16 14:29:21 GMT FROM: Sara Cutini at INFN M. Axelsson (KTH and Stockholm Univ.), S. Cutini (INFN Perugia), M. Arimoto (Kanazawa Univ.), N. Omodei (Stanford Univ.), D. Kocevski (NASA/MSFC), F. Piron (CNRS/IN2P3/LUPM), F. Longo (Univ. and INFN Trieste) and E. Bissaldi (Politecnico and INFN Bari) report on behalf of the Fermi-LAT Collaboration: We have searched data collected by the Fermi Large Area Telescope (LAT) on Sep 15, 2019, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S190915ak (GCN 25753). 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 null instantaneous coverage of the LIGO probability region at the time of the trigger (T0 = 2019-09-15 23:57:02.691 UTC). Coverage of the region started around T0 + 2.5 ks, and reached 100% cumulative coverage at approximately T0 + 3.2 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 1.1E-10 and 5.3E-10 [erg/cm^2/s]. The Fermi-LAT point of contact for this event is Sara Cutini (sara.cutini@pg.infn.it). 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.