M. Axelsson (KTH & Stockholm Univ.), E. Bissaldi (INFN and Politecnico Bari), D. Kocevski (NASA/MSFC), F. Longo (University and INFN, Trieste), and N. Omodei (Stanford Univ.) report on behalf of the Fermi-LAT Collaboration:
We have searched data collected by the Fermi Large Area Telescope (LAT) on April 25, 2019, for possible high-energy (E > 100 MeV) gamma-ray emission in spatial/temporal coincidence with the LIGO/Virgo trigger S190425z (GCN 24168).
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 a time, and "cumulative coverage" as the integral of the instantaneous coverage over time. Fermi-LAT had instantaneous coverage of ~37% of the LIGO probability at the time of the trigger (T0 = 2019-04-25 08:18:05.017 UTC), and reached ~98% cumulative coverage after ~4 ks. Due to the observing pattern of Fermi, the remaining area was not observed for more than 24 hours after the trigger time of the event.
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.
Two significant excesses (with TS>25) were found at R.A., Dec. = 278.7, -21.1 and 57.2, -28.2, respectively, but they are associated with known sources currently flaring (PKS 1830-211 and PKS 0346-27).
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 100 GeV for this search vary between 0.7e-10 and 3.9e-8 [erg/cm^2/s].
The Fermi-LAT point of contact for this event is Magnus Axelsson (email@example.com<mailto:firstname.lastname@example.org>).
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.