LIGO/Virgo S200128d: No counterpart candidates in Fermi-LAT observations

We have searched data collected by the Fermi Large Area Telescope (LAT) on January 28, 2020, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S200128d (GCN 26906). 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 an instantaneous coverage of ~35% of the LIGO probability at the time of the trigger (T0 = 2020-01-28 02:20:11.903 UTC), and reached 100% cumulative coverage after ~4.2 ks. We performed a search for a transient counterpart within the observed region of the 90% contour of LIGO map in a fixed time window from T0 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 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 1.4e-10 and 3.0e-09 [erg/cm^2/s]. The Fermi-LAT point of contact for this event is Makoto Arimoto (arimoto@se.kanazawa-u.ac.jp<mailto: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.