TITLE: GCN CIRCULAR NUMBER: 20499 SUBJECT: LIGO/Virgo G270580: Fermi-LAT search for a high-energy gamma-ray counterpart DATE: 17/01/22 20:36:18 GMT FROM: Daniel Kocevski at NASA/MSFC Daniel Kocevski (NASA/MSFC), Giacomo Vianello (Stanford), Nicola Omodei (Stanford) and Sara Buson (NASA/GSFC) report on behalf of the Fermi-LAT team: We have searched data collected by the Fermi Large Area Telescope (LAT) for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger G270580. The Fermi gamma-ray space telescope was passing through the South Atlantic Anomaly (SAA) at the time of the trigger (T0 = 2017-01-20 12:30:59.350). 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 data taking upon exiting the SAA at roughly T0 + 500 s. At that time the instantaneous coverage of the LIGO map was 18%, and we reached 80% cumulative coverage at T0 + 2.1 ks and 100% coverage within 8 ks from the trigger. We performed a search for a transient counterpart within the 90% contour of the LIGO map in the time window from T0 to T0 + 10 ks. We found only one significant excess, corresponding to J1048.4+7144 (associated with FSRQ S5 1044+71) which has been in a high state since last December (ATel #9928). 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 in neither of the two searches. 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.