TITLE: GCN CIRCULAR NUMBER: 25717 SUBJECT: LIGO/Virgo S190910d: No counterpart candidates in Fermi-LAT observations DATE: 19/09/10 15:05:24 GMT FROM: Magnus Axelsson at Stockholm U. M. Axelsson (KTH and Stockholm Univ.), L. Scotton (University and INFN, Torino), D. Kocevski (NASA/MSFC), N. Omodei (Stanford Univ.), E. Bissaldi (Politecnico and INFN Bari), F. Longo (Univ. and INFN Trieste) and M. Arimoto (Kanazawa Univ.) report on behalf of the Fermi-LAT Collaboration: We have searched data collected by the Fermi Large Area Telescope (LAT) on Sep 10, 2019, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S190910d (GCN 25695). 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 instantaneous coverage of 50% of the LIGO probability region at the time of the trigger (T0 = 2019-09-10 01:26:19.243 UTC), and reached 100% cumulative coverage after ~9 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. One significant excess (with TS>25) was found at R.A., Dec. = 57.5, -27.7 (the 90% containment radius is 0.4 deg, statistical uncertainty only), but it is associated with the known and flaring source PKS 0346-27 (0.29 deg from the best-fit position). 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 1.9E-10 and 4.7E-09 [erg/cm^2/s]. The Fermi-LAT point of contact for this event is Makoto Arimoto (arimoto@se.kanazawa-u.ac.jp). 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.