TITLE: GCN CIRCULAR NUMBER: 27199 SUBJECT: LIGO/Virgo S200224ca: No counterpart candidates in Fermi-LAT observations DATE: 20/02/25 10:25:01 GMT FROM: Lorenzo Scotton at CNRS/IN2P3/LUPM M. Crnogorcevic (Univ. of Maryland & NASA/GSFC), L. Scotton (CNRS/IN2P3/LUPM), N. Di Lalla (Stanford University), M. Axelsson (KTH & Stockholm Univ), E. Bissaldi (Politecnico & INFN Bari), D. Kocevski (NASA/MSFC), N. Omodei (Stanford Univ.) and F. Longo (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 February 24, 2020, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S200224ca (GCN 27184). 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. The Fermi Gamma-ray Space Telescope was passing through the South Atlantic Anomaly (SAA) at the time of the trigger (T0 = 2020-02-24 22:22:34.406 UTC). During SAA passages both the LAT and the Gamma-ray Burst Monitor (GBM) do not collect data due to the high charged particle background in this region. The LAT resumed taking data upon exiting the SAA at roughly T0 + 0.5 ks. At that time the instantaneous coverage was 5% of the LIGO probability map, and reached almost 100% cumulative coverage at approximately T0 + 6.4 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 + 0.5 ks to T0 + 10 ks. No significant new sources are found. We also performed a search which adapted the time interval of the analysis to the 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 2.6 e-10 and 4.5 e-10 [erg/cm^2/s]. The Fermi-LAT point of contact for this event is Lorenzo Scotton (lorenzo.scotton@lupm.in2p3.fr). 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.