With an instantaneous view of 70 % of the sky , the Fermi Gamma-ray Burst Monitor ( GBM ) is an excellent partner in the search for electromagnetic counterparts to gravitational wave ( GW ) events .
GBM observations at the time of the Laser Interferometer Gravitational-wave Observatory ( LIGO ) event GW150914 reveal the presence of a weak transient above 50 keV , 0.4 s after the GW event , with a false alarm probability of 0.0022 ( 2.9 \sigma ) .
This weak transient lasting 1 s was not detected by any other instrument and does not appear connected with other previously known astrophysical , solar , terrestrial , or magnetospheric activity .
Its localization is ill-constrained but consistent with the direction of GW150914 .
The duration and spectrum of the transient event are consistent with a weak short Gamma-Ray Burst arriving at a large angle to the direction in which Fermi was pointing , where the GBM detector response is not optimal .
If the GBM transient is associated with GW150914 , this electromagnetic signal from a stellar mass black hole binary merger is unexpected .
We calculate a luminosity in hard X-ray emission between 1 keV and 10 MeV of 1.8 ^ { +1.5 } _ { -1.0 } \times 10 ^ { 49 } erg s ^ { -1 } .
Future joint observations of GW events by LIGO/Virgo and Fermi GBM could reveal whether the weak transient reported here is a plausible counterpart to GW150914 or a chance coincidence , and will further probe the connection between compact binary mergers and short Gamma-Ray Bursts .