TITLE: GCN CIRCULAR NUMBER: 21667 SUBJECT: LIGO/Virgo G298936 : Fermi-LAT search for a high-energy gamma-ray counterpart DATE: 17/08/23 23:45:59 GMT FROM: Daniel Kocevski at NASA/MSFC D. Kocevski (NASA/GSFC), G. Vianello (Stanford), N. Omodei (Stanford), M. Razzano (Univ. Pisa & INFN Pisa) and S. Buson (NASA/GSFC) report on behalf of the Fermi-LAT team: We searched data collected by the Fermi Large Area Telescope (LAT) for a possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger G298936 (GCN #21656). At the time of the trigger (T0 = 2017-08-23 13:13:58.520 UTC , 525186843.52 MET), the Fermi gamma-ray space telescope was within the South Atlantic Anomaly (SAA). During SAA passages the LAT does not collect data due to the high charged particle background in this region. The LAT resumed data taking upon exiting the SAA at roughly T0 + 849 s. At that time 47% of the Bayestar probability map based on H1, L1 and V1 detectors (GCN #21661) was in the LAT field of view, and we reached 100% cumulative coverage within 4434 s after the trigger. We define "instantaneous coverage" as the integral over the region of the LIGO/Virgo 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. We performed a search for a transient counterpart within the 90% contour of the LIGO/Virgo probability map in the time window from T0 to T0 + 10 ks, and no significant new sources are found. We also performed a search which adapted the time interval of the analysis to the exposure of each region of the sky. No significant candidate counterpart was found. The Fermi-LAT point of contact for this event is Daniel Kocevski (daniel.kocevski@nasa.gov) 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.