The Fermi GBM and LAT follow-up of GW150914

Abstract

As the first detection of Gravitation Wave (GW) event arising from the coalescence of two stellar-mass Black Holes (BH) was announced by LIGO, a new era for astronomy began. Searches for electromagnetic (EM) counterparts of GW events is of fundamental importance, as they increase the confidence in the GW detection and help characterize the parameters of the merger. The Fermi gamma-ray space telescope has the best sensitivity to simultaneously observe a large fraction of the sky from 10 keV to more than 300 GeV, providing the unique capability of rapidly covering the entire probability region from a LIGO candidate. Here we present observations by the Fermi Gamma-Ray BurstMonitor (GBM) [1] and by the Large Area Telescope (LAT) [2] of the LIGO Gravitational Wave event GW150914, which has been associated to the merger of two stellar-mass BHs. We report the presence of a weak transient event in GBM data, close in time to the LIGO one. We discuss the characteristics of this GBM transient, which are consistent with a weak short GRB arriving at a large angle to the direction in which Fermi was pointing. Furthermore, we report LAT upper limits (ULs) for GW150914, and we present the strategy for follow-up observations of GW events with the LAT.